Re: Will he fight her?
Well, if it were me, and the cat didn't have a collar, I would take him in and have him neutered (and blood tested for Felv and FIV). Most places have low or no cost sp/neuter programs for strays. That way, he will calm down AND there won't be hundreds of unwanted kittens being born.
I would be certain that ALL your kitties shots are up to date. But as for them fighting, I think it's unlikely. They seem to be tolerating each other fine so far and that means they've accepted each other into "their" territory. That being said, you just never know. As long as your kitty is going outside, there's always the possibility that she will get into a rustle with somebody.