So the other night my boyfriend and was at one of his friends parties and was there all night, later that night I got a text from one of my very good guy friends saying that all night a girl (my friends step sister) was on his lap and they were together all night blah blah blah. When I told my bf what I heard he said it wasn't true and shit and that his friend was hitting on her not him but the guy friend who told me literally gains nothing from me knowing or not knowing about this so there's no need to lie but I just want some opinions on how I should handle the situation

Extra info-

The friend who told me is strictly platonic and has a long term gf

My bf didn't invite me to the party even tho it was a huge party and obviously not just a close get together