Then take another picture... after grandma moves, get more than one shot. I've done sports and events, I learned how to frame shots with both eyes open to be able to see if someone was heading into or out of the frame (and still wouldn't catch them every time and would end up with some that weren't usable, although at least they usually say - excuse me!). This happened in photojournalism long before anyone thought up the term photobomb, and having someone in the way in your shot most of the time probably isn't going to make for a good picture (photojournalistic or not).
I don't think it works because the hair blocks too much of her and takes up too much of the photo - if it had happened so that maybe it had been shot from a little farther back so you could see who it was, or from a different angle, it might have worked better. The family might like it because they know it's grandma and sometimes it's worth saving a photo if it has some significance to someone.