Would you like to know if you're significant other was cheating?

I work with this woman that is in a relationship with a very nice man. They have a 5 year old son together, live together, not married. Just recently this woman had decided to move on out the guy but soon found out she could not financially continue to live on her own, so the guy took her back in, no questions asked. Little does he know this woman is a down right nasty tramp that will have with any guy that will do her. She has even gone so far as to give blow jobs on the job. My question is do I tell the guy that she is cheating? I really want to tell him because I feel like I would want to know if my husband were cheating on me. What's even worse is this woman screens all HIS calls and questions everything HE does when SHES the one cheating! Any advise would be helpful.