What should she do?

I️ have a friend who says she’s madly in love with her current boyfriend , they’ve been going out for more than a year. At her current work place there’s this guy who constantly hits on her and she thinks he’s attractive. Unfortunately one day the guy snuck in a kiss. I️t was a kiss along the lines of a peck. She hasn’t told her boyfriend because she believes what he won’t know won’t hurt him but I️ believe other wise. What do y’all think she should do.