Umm... I feel that's a bad idea. People do all sorts of hasty/foolish/harmful things when they are emotional, and certain someone sure did got quite emotional there. Even if revealing that information could potentially defuse the situation, you have no guarantee of it. Envy, anger, and being "a tad on edge" from the latest events, it could lead to... unwanted turn of events.
Guess if you approached it with charisma, you could defuse the situation - with or without revealing that information to Clem... but as far as I know, Jonty doesn't have all that many points in it.
Besides, even if it revealing this information to Clem would defuse the situation right now, there is no guarantee she won't get upset over something else in the future, and then act rashly under influence of emotions... and reveal that information, plowing certain someone's life into chaos.

...Dam I'm sometimes yapping a lot.

I would think Clem be the type of person that honor certain secrets like that, but then again, I thought she would understand the difference between friendship and cheating.

The best excuse I can give clem is that, Jonty basically died and has been getting hurt throughout the time he has been with clem, I am guessing Clem thought Jonty blame clem for everything and wanted to be done with her, so he can be with the girl who recently saved his life.

Trauma's a hell of a thing, and from what I understand,(If I remember correctly) her so-called S.O. before Jonty gave her quite a bit... and Jonty is now having to deal with that baggage. Why does that matter here? Cheating must have been one of the things Clem caught her previous S.O. doing, and coupled with the trauma she has, I fear it could make Clem act pretty petty.

I think telling her everything would be the best course of action right now.

Besides Clem is trustworthy. :3