@zhjuan,
You mention you have a child of your own and the way you speak it seems you are a great parent very devoted to your child but sadly this isn't the case with everybody. The ability to have a child does not equate to always putting them first. Sometimes families who appear very loving and together on the surface have a lot of issues behind closed doors which is why cases of abuse sometimes go unnoticed by other family members and close friends. I am not saying this is what happened with your friend but please remember that the fact his mom cries and tells you she hurts doesn't mean her side of the story is the right one.
It is impossible to say what went on in their house that drove away your friend and his wife. It isn't just about her not liking his parents, he seems to have a lot of anger which might have been there since childhood. He might have his reasons for not telling you what his issues are and simply leave it as he wants to avoid conflict.
I don't think someone who grew up with in a loving, supportive family situation would turn away to the point of not even wanting to attend his parents funeral without reason. Maybe he has other issues of his own that have nothing to do with his parents but it is impossible to say without knowing if he is happy with his life now that he has broken off contact with them.