Cheating on his girlfriend?

This is my ex. We have been broken up for quite a while now. Haven't talked in a few months. Still not over him completely, but I digress. He randomly messaged me last week that he misses me and all this crap. Unfortunately I didn't save those messages. I saved these though, because on one hand I want to tell his girlfriend, but on the other hand I do not. He has been a pretty close friend to me. He was my only support system at a time, and I care a lot about him so I feel like I'm breaking his trust by going behind his back. But, on the other hand, his girlfriend doesn't deserve this and I know if I was his girlfriend I'd want to know. They've only been dating a couple months so I feel like it's better to rip the bandaid off now before she gets too invested? I don't know. I've always said I would tell the person being cheated on, but I'm honestly scared since it has to do with my personally.

Btw, NO I am not trying to break them up, and NO I will not get back with him even if he wanted to after this. I know in the first messge it seems like I would but that was right after he told me he missed me and everything, but I've come to my senses and do not condone what he is saying, nor would I want to be with someone who talks like this to other people.

***To all the people who've said to stop responding and block him, I HAVE. I was wondering if I should tell his girlfriend, but I've decided to leave it as is. Karma will come back for him. 🤷‍♀️