If I was to see my friends boyfriend out with another girl while my friend thought he was home studying before jumping to conclusions and calling her I would first investigate the potential situation.
I would start off by watching them from afar to get a feel of how they act towards each other such as friendly, affectionate, hostile etc… After having a somewhat understanding of what’s going on I would make my presence known to them by speaking to him. While talking to him I would also be evaluating his mood and the way he acted towards me being present. Once the conversation is over I would gather my information and call my friend and report to her my findings whether they be good or bad because I feel like she deserves to know and if it was me in that situation I would also want to know.
Another reason I would tell her isbecause even if he wasn’t doing anything sneaky my friend still believed that he was somewhere the he wasn’t which in turn makes him a liar. If I didnt tell her and she later found out from another source that I knew since the beginning it would affect our friendship negatively. She would begin to feel like i’m not her real friend and she would also feel as though she couldnt trust me. I know she would feel that way because that’s how I would feel if the roles were reversed.