What to do???????

Tell the GF! But be prepared in case you get a negative response. A lot or time girls who are insanely attached or into their boyfriends won’t believe you. It’s understandable but unfortunate. Like I heard from some girls there’s a dud in beta who’s dating a girl despite constantly flirting with other girls and leading girls on and his GF doesn’t seem to care or notice at all. So with that being said be prepared that some girls are really loyal to their BF’s to the point where they may not believe you or be as open to that convo cause it’s hard to hear that someone you love is doing that to you.
They’ll maybe get mad at you and not their boyfriend or excuse the boyfriends behaviour. I think it’s better that you just tell her because she deserves to know. Just calmly message her and take the chance.

Don’t ask on grank to start