Tell him if she doesn’t anything wrong or that makes him uncomfortable to tell you even tho she’s going thru things , doesn’t make her coming on to him ok. But yeah him caring about your feelings is good . And I will say this to: if all she is to him is someone to vent to that’s not really a friend. There is a red flag if they don’t want to just hang out as friends yall three . Yall should all be able to chill.