Oh.. I am so sorry to read this. I am sure Mother's day will be tough for her, maybe take her somewhere for the day. Maybe to the seashore or somewhere where people are not celebrating, in your face kind of thing, Mother's Day. Maybe, if it is a nice day, pack a lunch and just sit somewhere you can talk and enjoy the day.
If it is not a nice day, yikes... maybe go to a museum or just somewhere she will not have to be with all the other Moms celebrating their special day with their families.. Also, she may want to go to cemetary, if her Mom has been buried, and you might want to do that with her if she wants company..
It is a tough one, I wish I had more suggestions, truly when my Mom died, that first Mother's Day was so hard, but I had my children and I was their Mom so I kept myself in check..
Hope others have some good ideas here.