ex new bf cheating on ex

*No one* ever deserves to be cheated on, and the fact that you are concerned about her now means that you realize this. If you're absolutely, 100% certain this is happening (and you're also certain she doesn't know already), then you should tell her. It would help to provide at least some piece of evidence, even if it's just telling her something you saw. Something she won't be able to instantly refute.

No matter what though, don't press the issue. If she doesn't believe you, don't keep on her about it. You've told her, you did your part, and she'll be looking to see the signs whether she wants to or not. But since this will irrevocably alter her relationship with this dude, again, you need certainty.

You clearly care what happens to her, so I don't think you should let this just go by. If you're concerned about it being awkward for you to be the one, perhaps you can have someone else close to you and her do it for you without ever referencing you. If you can't be comfortable with that and really want to be the one telling her, then it's possible you may have subconscious ulterior motives. That shouldn't disqualify you from trying this, but be aware of it.

How ya doing, buddy?