"Hey, I think this will break if XXX happens (provide an example if you can). Can you please take a look before merging? Let me know if you need help".
I'd say that to any colleague (and hope they would to me) if I found a bug during code review, regardless of their team. I mean, it honestly sounds pretty routine and is the whole point of code review, no?
It doesn't mean I'm mentoring them or vice versa, and it's not a power play. It's just normal review in the line of normal work, peer to peer. It's fine that you're more experienced. We all start somewhere, and at some point, he'll be more experienced than you about some particular part of the code base, but your careful eye and constructive feedback is still valuable regardless. I don't see the problem.
Is there some context I'm missing?