WIBTA if I tell my ex-friend’s girlfriend, who still lives with him, that he cheated during the holiday break

So my ex-friend (let’s call him Josh) has been with his gf for over a year now (let’s call her Jess). Josh and I shared the same group of friends and we got along super well. We are all college students and the couple are living together with two of their friends as suite mates. I know Jess but we’re not (never were) close friends, not that it matters, but I was seen as her bf’s friend and I saw her as my friend’s gf.

After a year of college, Josh and I went back to our home country (he went first) and when I arrived he told me he cheats. At the time I told him that it wasn’t right but he continued and I did nothing to stop him (I know I’m in the wrong for this). I have the chat history of the cheating but I didn’t want to say anything because it would tear apart:

1. the leasing of their apartment as people would have to move out (their two friends who have nothing to do with the situation may need to find a new place because the monthly rent was shared. One less person would increase the costs.)
2. the friend group I was in (it would create a really messy situation, and I know it’s selfish)

I still see the Josh and the people of that friend group in college (coincidentally, not intentionally) and I don’t want to be seen as the guy who snitched even if it’s the right thing to do (it is selfish of me). I guess I just don’t feel comfortable being the one who causes all this trouble.

The reason why he is my EX-friend has nothing to do with the cheating and I’m not trying to break up the couple out of spite but instead, I’ve been feeling guilty about not saying anything for the past 4-5 months.

I’d appreciate some guidance on what to do or rather how to do it as, morally, I should tell Jess. WIBTA if I did? And what would be the best way to do it?