You know....I think I see this a little differently. It's still her anniversary, whether her dh is here or not. I think the dinner is perfect, but I might take it one step further and get her a card, and a small gift (some earrings, a framed photo that you took of the two of them, or something else), and tell her that this day should STILL be honored. Of course, give this to her when no one else is around, and let it be your little secret. I just think that with the death being so recent, that she might feel pressure not to commemorate the day, but not commemorating the day will be incredibly painful for her....so, if a friend commemorates it for her, maybe it will help her over this difficult hump.
I could be wrong....it is so difficult to know what to do in these situations, but letting her know that others miss him too on such an important day would be nice way to support her loss.