Advice

So currently my ex is dating the girl he cheated on me with and keep in mind we broke up in March and he recently started Tsing me again and he keeps saying things like he’s unhappy in the relationship and he doesn’t wanna be with her he’s only with her to pass time until he moves or finds someone new and even tho I dont like her for obvious reason I think what he’s doing is wrong and I’m not sure if I should tell her or not. Also the convo was on Snapchat and I thoidly didn’t save it so I don’t have proof unless I can think of a way to get in to repeat it. But I mean she did know about me when she was with him so bc of that I don’t know if I should be nice and tell her what’s going on. Any advice.