Oof. I'm so sorry to hear about your brother's trouble. It's a really rough spot to be in. I've been in similar situations and it always sucks. My solution has always been to back off, like Bee suggested. If he wants to stay friends with the guy then he should pick times/locations where the girlfriend is busy. Distance will
definitely help things. Feelings don't always need to be validated. Even if, in the moment, they can be overwhelming. Eventually they will fade.
There is absolutely
no way in which he can end up with this woman and you should warn him of that now so he doesn't get any ridiculous schemes in his head. Emotions can make people do
stupid things. If he breaks them up, which is a terrible idea and he should
not attempt it, the likelihood is she's just going to vanish from the social scene. People don't usually stay friends with their exes friends. So he'll lose her that way. Then there's the social implications of him doing that which will probably cause him to be ousted from the friend group. His safest bet is to just forget about it and move on. If he's able to play it cool maybe she's got a single friend she can introduce him to.
I wish him luck because this is hard