First, let me clarify some distinctions; a Marine is certainly not a Soldier. Soldier is Army, Airman is Air Force, and a Sailor is Navy.
Second, the story is false. As someone with an 'inside scoop', the President wouldn't (and cannot) 'relieve' an enlisted military personnel on the spot.
The closest I could find to this story is this article.
A 26-year-old sergeant and Iraq veteran is in the process of being
dismissed from the Marine Corps after making "political statements"
about the commander-in-chief on Facebook.
Such comments are considered detrimental to "good order and
discipline" under the Uniform Code of Military Justice.
It is just that, enlisted personnel are under a contract with the government to serve the time period they signed for unless violating a Punitive Article of the Uniform Code of Military Justice, medical separation, or a few other special circumstances. And for every circumstance, there is a board of high-ranking, competent personnel who make the decision to separate an individual. If the President had in fact said such a thing, it would be in direct violation of the Marine ethos of being able to correct anyone, regardless of rank (In the correct manner, of course).
I'm going to chalk this up to President-bashing, which has been gaining increased momentum over the past decade. The likelihood of the President of the United States (Whom is ALWAYS shown in the world's eye) would wear the American Flag pin upside-down, which is disrespectful of our country in almost every way. He has teams of people to look over his appearance before stepping out and into the public's view.
If anything like his had happened, it would be all over the military press (Yes, we have a press).