TL;DR The ratio of ADHD to non-ADHD college graduation rates is approximately true according to the cited source. The generalization from source to claim may not be accurate.
The Medscape article references a study it refers to as the "Milwaukee Young Adult Study" for the claim. In this early longitudinal study, 239 children (158 hyperactive1 and 81 control) around the Milwaukee, Wisconsin (US) area were surveyed for "cognitive, adaptive, family, peer, educational, occupational, financial, sexual, driving, and health-related variables" in childhood, adolescence, emerging adulthood, and young adulthood. The results of the study were not published in a single paper. Instead, many papers were published on this single study, focusing on specific aspects such as driving violations, drug use, or STD prevalence. The Milwaukee study was co-led by Russell A. Barkley.
Russell Barkley coauthored a book about ADHD that included the college education results of the Milwaukee study. Page 254 shows 9.1% of hyperactive young adults diagnosed with ADHD graduated from college. In contrast, 68.0% of the young adults in the control group graduated from college. These statistics are true for a small population of 239 children not representative of the US population surveyed when they were between 19—25 years old). I would assume 19 year olds cannot be expected to have graduated from college. Thus, there may be some systematic error introduced here.
While the absolute percentages in the Medscape article are incorrect, the relative ratios reflect the results of the study. The Medscape article lists graduation rates of 5% and 35% for the ADHD and control groups, respectively. Thus, people without ADHD graduated at a rate 7 times higher than that of people with ADHD. For the Milwaukee study, the ratios are very similar (different by ~1%).
How Medscape obtained their numbers: According to the US Census Bureau, 33.4% of Americans had a bachelor's degree (i.e. college degree) in 2017. Medscape rounded this number to the nice, whole number of 35%. To reflect the 7-times graduation rate of the control group, Medscape used 5% to represent the graduation rate of people with ADHD.
Do people with ADHD have a college graduation rate of 5%?
I couldn't find a study representative of the US population where people with ADHD had a college graduation rate of 5%. The Milwaukee study is the second largest I could find. Another large study with 352 participants, the UMASS study, found people with ADHD were half as likely to graduate when compared to a control group without ADHD (page 252 here). Then, instead of 5% vs. 35%, we would see 17.5% vs. 35%.
Is there evidence that people with ADHD actually graduate at one-seventh the level of those without (5%)?
According to the Milwaukee study, people with ADHD were 7 times less likely to graduate from college. Therefore, there is evidence for this.
From Medscape: "For those who do go to college, those with ADHD have a lower college graduation rate (5% vs 35%)."
Yes, this is true according to this one study.
1 I purposefully use the term "hyperactive" because some of the children in this group were not officially diagnosed with ADHD. Thankfully, the study separately analyzed the children in the hyperactive group with ADHD and the children in the hyperactive group without ADHD.