The claim grossly misrepresents the details of the law
TL;DR $50,000 is the maximum amount someone could be liable for due to egregious and repeated discrimination on account of gender identity, and the situation described in the claim is extremely unlikely to qualify.
First things first: this answer is predicated on the assumption that a "No" result on the Massachusetts Veto Referendum known as Question 3, which will be voted on on November 6, 2018, would have the effect of repealing Bill S.2407 and nothing else. The best source I can find for that claim is the relevant Wikipedia page, but I haven't found any conflicting information either so I will be taking it as fact.
The full text for Bill S.2407 is available for download from the MA Legislature site. It's very short, so I was able to read the entire thing. The entirety of the bill is adding the phrase "gender identity" to various lists of "things you are not allowed to use for discrimination".
From here we can address this part of the claim:
any man who says he is a woman can enter a women's locker room, dressing room, or bathroom at any time.
Definitions for terms used in bills are listed in General Laws, Part I, Title I, Chapter 4, Section 7, and "gender identity" is defined in the Fifty-ninth clause (emphasis mine):
"Gender identity'' shall mean a person's gender-related identity, appearance or behavior, whether or not that gender-related identity, appearance or behavior is different from that traditionally associated with the person's physiology or assigned sex at birth. Gender-related identity may be shown by providing evidence including, but not limited to, medical history, care or treatment of the gender-related identity, consistent and uniform assertion of the gender-related identity or any other evidence that the gender-related identity is sincerely held as part of a person's core identity; provided, however, that gender-related identity shall not be asserted for any improper purpose.
Thus if someone claims a certain gender identity for the purpose of doing anything "improper" in a women's bathroom, then any protection granted by S.2407 doesn't apply and repealing S.2407 would not change the situation.
For the other part of the claim, especially the fifty-thousand dollar fine, requires some more work. I'll go over the laws affected by Bill S.2407.
The first law affected by S.2407 is General Laws, Part IV, Title I, Chapter 272, Section92A. Here there are two important parts:
An owner, lessee, proprietor, manager, superintendent, agent or employee of any place of public accommodation, resort or amusement that lawfully segregates or separates access to such place of public accommodation, or a portion of such place of public accommodation, based on a person's sex shall grant all persons admission to, and the full enjoyment of, such place of public accommodation or portion thereof consistent with the person's gender identity.
This does not account for "other customers" or "other persons with admission to those places", so this law does not apply to the majority of people at all, let alone the majority of situations where one might "see something suspicious". Additionally, the maximum punishment for a violation is given:
Any person who shall violate any provision of this section, or who shall aid in or incite, cause or bring about, in whole or in part, such a violation shall be punished by a fine of not more than one hundred dollars, or by imprisonment for not more than thirty days, or both.
The fine is far short of $50,000.
The other law affected by S.2407 is General Laws, Part IV, Title I, Chapter 272, Section 98. My reading is that it defines additional penalties for violating Section 92A rather than defining any separate offence. It similarly has a maximum penalty:
a fine of not more than twenty-five hundred dollars or by imprisonment for not more than one year, or both, and shall be liable to any person aggrieved thereby for such damages as are enumerated in section five of chapter one hundred and fifty-one B
$2500 is significantly more than the $100 from Section 92A, but much less than the claimed $50,000. However, it also says they might be liable for additional damages. The relevant law in this case is General Laws, Part I, Title XXI, Chapter 151B, Section5, which defines civil penalties and also imposes limits on the damages that a person could be liable for (emphasis mine):
If, upon all the evidence at any such hearing, the commission shall find that a respondent has engaged in any such unlawful practice, it may, in addition to any other action which it may take under this section, assess a civil penalty against the respondent:
(a) in an amount not to exceed $10,000 if the respondent has not been adjudged to have committed any prior discriminatory practice;
(b) in an amount not to exceed $25,000 if the respondent has-been adjudged to have committed one other discriminatory practice during the 5?year period ending on the date of the filing of the complaint; and
(c) in an amount not to exceed $50,000 if the respondent has been adjudged to have committed 2 or more discriminatory practices during the 7?year period ending on the date of the filing of the complaint.Notwithstanding the aforesaid provisions, if the acts constituting the discriminatory practice that is the object of the complaint are committed by the same natural person who has been previously adjudged to have committed acts constituting a discriminatory practice, then the civil penalties set forth in clauses (b) and (c) may be imposed without regard to the period of time within which any subsequent discriminatory practice occurred.
If an employee of a place of public accommodation denies access to that place on account of "suspicions", and those suspicions are founded upon the alleged gender identity of a person, and the case goes to court, and that person is found to validly have the gender identity in question, then the employee could be fined up to $2500 and/or jailed for up to 1 year. Additionally, that person could then file a civil suit against the employee, and if the employee had committed 2 or more practices deemed discriminatory within the prior 7 years then the employee could be liable for a maximum of $50,000 in damages.
In other words, there is a way in which a person could be required to pay $50,000 because of their actions, but if S.2407 were repealed they would no longer be required to pay that $50,000. That amount is not a fine as claimed, but I doubt many would tremendously care about the distinction. However, the claim VASTLY under-represents the requirements for that to happen, specifically:
- The act of "see something suspicious and say something to authorities" would have to qualify as discrimination or refusal to "grant all persons admission to, and the full enjoyment of, such place of public accommodation".
- The person doing that act would have to be "An owner, lessee, proprietor, manager, superintendent, agent or employee of any place of public accommodation, resort or amusement that lawfully segregates or separates access to such place of public accommodation, or a portion of such place of public accommodation, based on a person's sex", and taking the act against someone attempting to make use of that place.
- The person being reported would have to prove in court that they were acting in accordance with their "sincerely held" gender identity, and provide evidence of that gender identity.
- The person who did the reporting would have to be found to have discriminated unlawfully multiple times within the last 7 years.
- The civil court would have to assign the maximum possible damages for the discrimination.