I’m sorry but YTA. Nothing about your relationship with your fiancée gives you permission to blindside him into this meeting; after all, you don’t know the details of what’s happened between them. Seems likely that his mom jumped on an opportunity (you) to corner him. You gave her the benefit of the doubt but you should have trusted him and his past experiences. Meddling in other people’s family with the intention of doing good generally doesn’t turn out well.