u/-mares

My friend just told me he cheated on his girlfriend. He broke up with her a few days after that happened however when they were parting ways he didn’t tell her he cheated. We had an argument whether he should tell her for the sake of telling the truth. As we do not agree he decided to leave our groupchat and doesn’t wanna talk to any of us in the friendgroup. I’m wondering if his reasoning that it’s better for her to not know (as it was her first relationship) because it will (citing him) traumatize her for next relationships and hurt her. Shouldn’t she know though?