Should I tell a girl her bf cheated

Hi,

I was in a relationship for two years, then he broke up with me, treated me horribly for a few months, got into a new relationship and blocked me. A month after he texted me again looking for sex, and he kept saying things that hurt me and would have hurt his new girl too. He basically treated me like a sl*t and cheated on her, even though it was just through text.

It's been a month since then, he blocked me again right away and I've been wondering whether I should send her the screenshots or not. I guess I haven't so far because deep down I still cared for him and I didn't want him to hate me.

But some days ago my grandmother died, and it hurt so bad, and I realised I can't even tell him because he blocked me, and it made me feel so mad that he gets to enjoy his life and his girlfriend while I lost one of the most important people to me. It just feels like Karma isn't getting him and I was thinking that maybe I should get him, I should expose him to his girlfriend and finally create some bad into his life.

I know it's petty and negative and I just don't know what to do. I want to tell her because it's the right thing. I don't want to tell her because he'll hate me. I want to tell because he deserves something bad. I don't want to tell because I lost my grandmother and I barely care about anything else now.

Any advice?