E.W.
One thing is that you can not ignore the elephant in the room and after all, she is still his mother. This will be an especially hard day for her-if she still needs to cry, let her cry. We always hear it is the worst thing that can ever happen to a Mom and do not deny her her morning.
If the family can handle make it a day all about him and bring some laughter for everyone. What might he have done for her on that day? Honor the two of them and do not be afraid to bring him into the fold.
It is awfully tough to ask her what SHE wants to do. She probably has no idea and probably doesn't want to make any decisions right now. I was goning to suggest the samr thing as previous. Take her nice like Arborgate and let her pick out a tree or rose to plant for him. Could be in her yard then perhaps a small plaque honoring him.
E. W.