Should I tell her her mans cheating

So about a year ago I met this kid through work. Never talked in person once but I quit that job and he found me on insta and messaged me a little while after I had quit. So his girl and him weren’t dating and had sex and she got pregnant. So they began dating. Now when she was giving birth he was asking for nudes. He said he wants to fxck me and always asks for nudes from me. Should I message his gf on fb and tell her he’s cheating or leave it be since I don’t really know other of them.