Should I tell my cousin that her boyfriend cheated on her? (Read description first)

So my semi-friend (who is on my golf team & is only my friend because she was the first nice person to me when I came to my school at the beginning of last year) has informed me that she has sex with a guy. She said that it was let say his name is Bob, so she had sex with Bob. But Bob is my cousins boyfriend...my cousin & Bob do not have sex or do oral or anything. They just kiss & make out. I just moved back to my mom's home town. So my cousins & I aren't close. Should I still let my cousin know what I have found out? Or let the golf season pass so I don't have to worry about having beef with one of my teammates?