When I injured my thumb pretty badly I used a thick marker with a few rubber bands wrapped around the end (to keep it from slipping around as I pushed down) to open it. Then I lost that one day and rummaged around my kitchen and grabbed this wine bottle stopper that worked perfectly for the same purpose. It had a rubber end (the part that would seal up the bottle) so it didn't slip around and the top was nice and thick and wide so easy to grip with the palm of my hand so I could use the strength from my arm to push in instead of having to use my hand too much. It looked kind of like this one:
I remember thinking at the time that it stunk that there wasn't any device specifically to make that easier. There are so many older people with arthritis that have that same issue. My dad has arthritis and I'm not sure he could do it either but when he's out with my son my mom is always with him. I've heard it is an issue from several people who have parents and grandparents who care for their kids.
If I were her, before buying another seat, I think I would make a trip to the hardware store, bring the seat in and try to find a helpful clerk that would let you just try a whole bunch of options until you find something that works. You know how men love to fiddle with stuff and find solutions to problems like that!
