The hookup happened a good while ago but I just found out about the lying. He initiated it and told me he was single (recent breakup). Turns out he wasn’t actually single at the time, they were in a fight that lead to breakup later, and now the ex gf is mad at me for not telling her about the hookup. I wouldn’t have reciprocated if I had known he wasn’t actually single and I didn’t mention it to her because I was told by him that they had separated and therefore it wasn’t her business. I feel so shitty that I accidentally homewrecked and hurt the gf even though I was lied to.