Ditto on all the "let Grandma choose" suggestions!
BearIG suggested doing a gift certificate. Maybe you could find a toy sewing machine, wrap it up, and when she opens it have a personal note about your plans. Or perhaps you could do up a notions basket of items that she can use with her new machine! Then the two of you make plans to have FUN shopping together.
One suggestion I will add if... if it is in your budget, encourage her to buy a machine one step above what she feels she wants/needs. So often we sewers choose what is right for us at the moment, forgetting that we are still developing our skills, and as we do, a machine offering "more" will be in our sights! Yes, Been there, done that, have the T-shirt!
The other thing though, consider that Grandma may not have interest in a new machine and may be quite happy with what she has! Respect her wishes, if so.
There are SO many ifs ands and buts about machines. Two people of the same skill level, often don't agree on what they want in a machine. Thus why we are steering you towards not buying on your own.
Good Luck and treasure the special time this can be with your Grandma!