What do you think my friend should do? Relationship Question

So say you were messing someone for almost a year, best friend to be exact, and then all the sudden your like “hey i like this girl, lets stop having sex,” so you start dating this girl meanwhile still messing around with the best friend, not sex but receiving head, mutual masturbation, naked things people in a relationship shouldn’t do, and then one night you tell the girl you really don’t know why your dating this one girl, because you think you can do better. Do you think it is wrong for the girl to keep messing around with the boy, and for the girl not to feel bad?