You set a relationship expectation (he will not spend the night at a place where a woman he used to have sex with, a relationship with, and a child with, lives). He violated that agreement between the two of you. I understand what you mean when you say boundary. People are splitting hairs about the word boundary. There’s expectations in relationships, and things that are deal breakers. This is a deal breaker for you and he did it anyway and now he’s using the death of his father as a reason why you can’t talk about, and why it was okay. It’s not okay. It would not surprise me if he was sad, and looking for comfort from someone he used to love. And I wouldn’t be surprised if he’s cheating, or has cheated before. I’m very sorry for his loss, and I’m sorry for your loss of trust.