OP, I hate to say this, but if he's bad enough to cheat, and propose in front on your mom, how do you know he wouldn't go to your mom further to manipulate you? What if she did this, then he told some BS lies to the sick mom and hurt everyone more? Maybe saying nothing is better to protect yourself and your mom, but know in your heart you are done and spend no energy on the guy, emotionally check out and don't interact as much as possible. If he asks what's wrong, just say you're grieving. Then leave when the moment is right
You are not responsible for him being an immature, insecure pos, and you need to get safe
OP, I hate to say this, but if he's bad enough to cheat, and propose in front on your mom, how do you know he wouldn't go to your mom further to manipulate you? What if she did this, then he told some BS lies to the sick mom and hurt everyone more? Maybe saying nothing is better to protect yourself and your mom, but know in your heart you are done and spend no energy on the guy, emotionally check out and don't interact as much as possible. If he asks what's wrong, just say you're grieving. Then leave when the moment is right