Would you want to know that your bf is cheating through the girl he's cheating you with? Or should I let her be..

Problem/Goal: Encountered a guy that's a cheater and I didnt know

Context: Just recently encountered a guy who said he was single. We've been flirting and doing stuff for months now. I didn't know that he was in a relationship, he was so good at hiding it. I found out through his other random social media that he accidentally sent me and he thought I didn't see but I saw. I immediately looked at it and then boom. that's when I found out, matagal na pala sila. if I knew, I wouldn't have done it. I immediately cut it off and blocked him. but I feel sorry for the girl. I dont know if i should tell her or just let her be. They are in an LDR that's why he cheats so easily. I feel so guilty because I've been cheated before and I hate cheaters. I felt disgusted when I found out and the feeling of this burden is really eating me up.

If kayo ba yung girl, would you want to know that your LDR bf is cheating on you?

Previous attempt: Wala pa

PS - Idk if this is the correct flair.