IME vortex pieces and the FC710 (with the little ball) require a pretty good draw to get started, though they aren't too bad once everything is going in the right direction. Could see this being a problem for your elderly friend.
You'll probably be best off with something that has very little diffusion and a bigger hole(s). So instead of a matrix style perc, or what is sometimes referred to as a "tire" perc on DHGate, something like the FC UFO that has wider slits, or something that is wide open. Wide open as in just a straight open tube into the can that would be submerged in a little water. I've seen a few of these posted here and there recently, but can't recall numbers.
Smaller volume is probably better, but more importantly (especially if the user doesn't mind taking two draws on the glass for one hit) is something that can "bubble" or filter, with very low water level. Generally, the more water you have to go through, the more draw it will take. Same for restricted or intricate diffusion.
What kind of vape will be on it? Restriction in its draw will have an impact, too.
Depending on what type of vape, a simple PNWT type bubbler might be best. They don't generally take too much of a draw to use, though they also don't offer too much cooling or moisture.