Can I tell my ex's new partner about his potential infidelity?
Don't you think it's best to leave it alone?
Just trying to be a good friend.
I don't think you should involve yourself in their relationship. Plus you said "potential infidelity", which really means nothing.
I just want them to talk about it. They deserve to know the truth.
You would be interfering in a relationship that has nothing to do with you, and just because he has strayed in the past does not mean that he'll do it in this relationship. You would be a bad friend if you tried to prevent them from dating by telling the bad things your ex has done in the past.