secret from a girl ive concidered a friend

Long story short i had sex with the guy a girl friend had a lil crush on but she knew he didnt like her bc she had got mixed up in some drama with his bestfriend who was married with kids plus he let them know also he just was a hi bye friend & now they don't talk at all neither of them but i do & i somewat feel bad i haven't told her but then again im not sure if that's something i need to let her know.. she isnt my bestfriend nor my greatest friend bc i barley know her & what i know of her is she single & loves to mingle so im not sure if im wrong for that also i have plenty of proof where she's made me look like a bad guy to them just to make excuses to talk to them but like i said idk her like that ive only known her for 5months & in the 5 month's ive actually spent time n shared party events & bdays invited her to like 4 times & not once did she invite me to anything besides to acompanion her for a hook up shes scared to go alone now do to her past experiences but bc i call her a friend n know she had a crush on him im not sure ? what yall think about it ?