I vote tell her! My best friends caught my ex - boyfriend with another girl and failed to tell me. When my ex decided to tell me, I was crushed that they didn't tell me. My relationships with them have suffered for it. I will never trust them again. Since you think your source is reliable I would tell her!
I think this is a really good point. It kinda feels like you're damned if you do, damned if you don't, ya know? But I think you should tell her. I think you'll sleep better at night knowing you told her and that you did the right thing. Telling her at least gives her the option of thinking about it and by withholding the information, you'd be denying her the opportunity to make the choice if she wants to stay with him or not.
If you do decide to tell her, good luck to you. I like the way of telling her that you came up with--that you have no way of proving it, but that you want her to have the info and she can do with it what she wants. She's lucky to have a friend like you who cares so much about her.