I donated a kidney to my husband 11 years ago. If they are not too far from you, the best thing they will appreciate will be:
Rides to the dr. At first he will have to go every other day and since none of them will be able to drive much, a ride is much appreciated.
Help with house chores: cooking, cleaning, laundry, shopping, help with kids if they have them, etc.
Don't send flowers, since he will be immunosuppresed and why bring possible sources of contamination if you don't need them.
Help with pets if they have them. As bad as it sounds, he may be better off if the dog or cat is taken away for a little while. Again, because of the strong immunosuppresion he will be at first. Animals can be re-introduced shortly after.
$$$ Even if they have good precription coverage, if they have a co-pay bills add up. My husband was in at least 7 different medications and at $21.00 co-pay each per month, we needed all the help we could get.
While they are out in the hospital, pay for a professional to clean their house so he comes back home to a very clean house. That will help her a lot too. Not with the immunosuppresion, but knowing that she doesn't have to do it.
I know the surgery has changed a lot since I had it. Recovery period is much shorter for the donor and the recipient feels so much better after the transplant that the recovery seems easy, but help and support I am sure will be very well appreciated, much more than flowers or candy.
My mother gave us the best gift. She came from Puerto Rico and stayed with us for a month. She ran the household for that time. She made sure every one left their shoes outside to keep the house clean, and took the kids to school and picked them up.
I wish your family the best of lucks and please, keep us posted.
Maria