You won't score any brownie points throwing her a BD party, especially since she has said she did not want one. If she wants to mourn the fact she got to live 50......then let her, by all means.
I would talk to D14 about it, and see if she wants to take her mom out to eat. Then you could take her to one of those places where the waiters all gather around your table and sing Happy Birthday!
If there is anyone there that knows her.......I'm sure they won't look to see who has a BD.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!