The nerve of some people. If it were me... My mother-in-law would never be receiving a gift ever again after the initial incident. It's honestly baffling how someone could possibly be so entitled about something that is being given to them as a gift.
There's got to be other reasons why the husband was "unable to be present" at his mother's birthday. If I were him I'd make sure to have a scheduling conflict every year.
That in mind... It's cruel to send your wife in your place to your mother's birthday. Either you both go or no one goes.
This isn't the first we've seen of entitled in laws and one can imagine that it won't be the last.