I'll be straight, for older folks, the electric stuff is a better option. Not cordless, those are worthless.
An extension cord reel is easier to handle. As you get older, patience must replace brawn so dragging the cord around becomes the better alternative, as much as it's a PITA. Even if you have to chain 2 of them together to get where you are going.
I have a 150ft extension cord reel that weighs probably 10 pounds and it's very easy to reel back up and deal with. This is what my dad is doing now.
If you insist on using the gas model, my first inclination would be to look for a single point tactical sling. It would be enough to take the weight off of her arm. It might require putting a stud somewhere in the plastic for an attach point, you would have to experiment with some tape or something until you found the right center of gravity, or at least the point that allows the most freedom of movement.
If this point ends up being in the handle area, that would be ideal and you could simply use velcro straps and improvise from there... and even sew the sling to the velcro strap.
This may not be what you are needing, and I'm all but certain anything like this that you do will be somewhat awkward, but the photos might spark other ideas for you.