It looks like he is helping her get over her grief and that's the important thing right now. She has a kitten to snuggle with and wash, and hopefully you can keep him and they will be together for a lifetime. Yes, he will need to be neutered, but that can certainly wait a few weeks. If she does go into heat, watch him carefully and see if he shows any interest. If he starts trying to figure out how to breed, you will have to separate them and that would be a good time to get him neutered.