Absolutely enjoy the time you have with her and that you are able to do this!
Do not let guilt destroy the day and the wonderful thing you are doing. My father died 6 months after my 44th Birthday and the thing I remember most was the wonderful Birthday Card he sent me and what he wrote to me. He was an elder for many years and eventually left, but realized the importance in acknowledging the wonderful people in his life. A birthday is really a celebration of the joy of having this person in your life and saying I'm glad you are here!
My own mother, raised a DEVOUT witness from birth, always wished us happy birthday, and also always called her own brothers and sisters (all devout) to send birthday wishes. In fact they all did, and one was of the anointed.
So you may enjoy the day guilt free.
Send my best birthday wishes to your mom!