Should he be told or should it be left up to fate?

My ex fiance left me for someone else after 7 yrs. with no warning. He has been seeing this person for about a mth. now- I know what they say about rebound relationships very rarely working out but, he seems to be quite infatuated by her & moving the relationship right along. This girl lives 3 doors down from a very good friend of mine which I'm sure she has no clue of. Recently, my friend has seen her on numerous occasions with another guy hugging & kissing him when my ex is not around. My ex lives almost a hr. away from us so if she is not at his place he has no idea what she is up to. I know that I should not be concerned with what is going on in his life since he has left me but, I'm still in quite a lot of pain over all this & I still love him very much & it gauls me that he ended our relationship for her & she is not even being faithful to him like he thinks! Should he be told by someone, not myself of course that she is messing around on him or should I just sit back & watch her do to him what he did to me?