I know a bad secret about my ex's new girl. Not sure what to do?

him and I have been no contact outside of work for a while now

i know something about his girlfriend that will probably make him break up with her.

the few of us that know are afraid she will do something like this again and he'll find out. he seems the type that will be extremely depressed.

we care about him. the one person who could probably say something doesn't seem to want to tell him. I feel as the ex that I definitely should not say anything.

but I don't know if I should stand by and let him find out for himself. it seems cruel and I definitely would want to know asap if I was in that situation.

my other friend who knows about this suggests I should rekindle my friendship, so that I'll be there when things go down. I don't know how to approach him at work or if I should ever find him on AIM.

this has been on my mind every day now. please help.
None of you say anything. He'll find out eventually.
Vote A
Tell him you're there for him and you're ready to be friends.
Vote B
Tell him the secret and be there for him as a friend.
Vote C
Select age and gender to cast your vote:
GirlGuy
Updates:
+1 y
I know it seems silly I didn't just say the secret. Sorry, just paranoid. She cheated on him early on in the relationship. She might do it again.
+1 y
Out of the 3 of us that know, the guy in the group is thinking about whether to tell him or not. My friend and I are not going to say anything. I really do not feel it is my place as the ex.. And my other friend was never close to my ex. He might not tell
I know a bad secret about my ex's new girl. Not sure what to do?
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