My best friends social anxiety

So recently my best guy friend has started going out with this girl, were still in school and she's in the first class but he's in the last class.
They don't see each other but when if they do he doesn't really talk, then all he does afterwards is have a strop at me, because he feels socially awkward around her, he's starting to lose a lot of friends because she's a very controlling girl (she told him to stop being friends with me because "we hang out too much" ) I don't know who to talk to about this or how to help him.
Should he stay with her or should he dump her? Help please 😔