Help me to tell one of my friends

How can I tell one of my friends that her boyfriend is cheating on her?

just tell her . She has a right to know. Call her up and say ' hey , I got to tell you something . I'm sorry but dont cry . your boyfriend ( his name) is cheating on you .'

You can do it! She will appreciate it, no matter how hard it is to hear it. If you are really a good friend to her/him then you owe it to them to tell her! She will love you forever for it

That's a tough one as you don't want to loose her friendship. She may not even want to beleive you if you did tell her, which would put a strain on your relationship with her...
Does the guy know that you know he's cheating? If he does then he doesn't really care about getting caught...maybe he wants you to tell her because he hasn't got the guts to break up with her.
I can't tell you what to do but for me,I would just be there for her when the breakup happens because she is eventually going to figure things out on her own...she'll need a friend when that happens.

Pose it as a hypothetical situation. Ask her what she would do if loverboy ever cheated on her and if she thinks she would be able to tell by his behavior. That will get the ball rolling and emotionally prep her before the big blow. Then make sure she doesn't do anything rash, especially with a baseball bat or lighter fluid.