I guess the part of the OP that would really sting me, is that the son divulged that 'they talked about it and decided to just keep it just her family'. Ouch.
No reason a son can't understand that his mother has feelings unless he's heartless. Seriously.
Has absolutely nothing to do with him choosing his wife over his mother or his mother over his wife. That's not the case at all.
Unless the wife (DIL) doesn't like the mother in law -- you would think she would know the hurt being caused to her husband's own mother. (Of course, she does!)
I'm not talking about this particular situation, I'm talking in general.