Unfaithful boyfriend

A really good friend of mine is having a relationship with a boy for one year. The thing is that someone told me that he had been with other womens at the same time, I don't know if it's truth, but in my opinion I think that probably could be truth. Have I to tell to my friend or not? What I have to do? ..... I don't want being in a trouble with them and I don't have any evidence.