Helppp

Ok so so my friend and my boyfriends friend are dating each other but recently my fd friends ex has been going to his house lately to "hangout with his mom" but my friend only thought it was only for one day but it's was actually 3 days my boyfriends friend told me not to tell her but idk what to do Bc she is my friend and he is my friend , and then again she didn't tell me that she saw my BF face timing some other girl a few months back until weeks later . What should I do !!