As others have wisely suggested, just create a “tether” of some kind which will dangle below the bottom of the gate: rope has been mentioned but I would prefer some woven webbing material rather than actual rope because webbing is smoother to grab than any rope I’ve seen. Ideally it would be attached to whatever handle is on the low end of the gate intended for pulling the gate down. But as someone also mentioned, that would require tossing the tether inside before fully closing the gate, which might be a bit of a gymnastic move; do-able but annoying.
I have two refinements to the tether idea. First, and I don’t like this very much but it would be simplest, is to attach one end to the tailgate inside pull down handle, and the other to something on the interior such as seat frame, or whatever is there. Adjust the length to keep the gate from rising to its full height, low enough that your mom can reach the handle.
What I visualize though is a SHORT webbing strap attached securely to the tailgate handle, just long enough for mom to reach comfortably, with a bungee cord attached to the free end and that anchored somewhere inside. If you calibrate the length just right, and pick the right anchor point inside, the bungee will stretch when the gate rises, and then pull the webbing away from the bottom of the door sill when she closes the gate. If you use ordinary bungees, be sure to squeeze the hooks closed with pliers so that they remain attached. Good hardware and building supply stores will sell webbing and “shock cord” in bulk. Then, learn how to tie a “two half hitches” knot, and use that to make the connections.
Two half hitches knot