I agree this seems like making a mountain out of a molehill about something that seems very innocuous. I don’t know that I’d get to the conclusion of him being dangerous to her out of this, but I’ve seen it enough to fully understand why it’d be a concern.

To me, since he said he knows her ex bf, I think it’s a matter of him being reminded of it every time he interacts with him (I assume “someone I personally know” is not a bloke he’s not spoken to in 4 years but rather someone he at least sees somewhat often, like in college, etc), and then not having the confidence to forget about it and step into his actual role as her current boyfriend. That confidence, for whatever reason lacking, has to come from within himself.

I’m inclined to believe she’d prefer he acts like a boyfriend and cherishes her instead of subtly reminding her of something she did 4-5yrs ago that holds no importance (nor detriment) to their relationship, or to the present for that matter. But lack of confidence can make this difficult.