Oh God what a situation. I don’t see sadly how the relationship with this guys partner can continue. What is really obvious is that Daddy was married when this occurred and his daughter would hate her Dad for his betrayal of her Mum- understandably. Be the death knell for Daddy’s marriage if his wife ever found out. Options- both men can keep the whole thing from their partners. Wrong on many levels. Or the relationship has to end. Imagine family gatherings with that secret in the air. Impossible. What a truly sad and terrible situation.