We can play "what if" all day long, but the key point is that your friend wasn't involuntarily committed for treatment or declared to be mentally defective by the courts. That's the bar we have right now before someone becomes a "prohibited person", and it's pretty darned high, just as it should be.
Actually, the bar is only that high in part of the country. In the First, Second, and Sixth Circuits, even a temporary hearing for observation can cause you to lose your rights. There is a case in the Second Circuit where a man lost his Second Amendment rights forever just on the word of two doctors. No hearing.
S.480 by Senator Graham actually corrects the law so that the high standard you mentioned applies across all of the United States.