It's a pretty easy boat to dive from, all things considered. The other alternative would be to find a better strengthening regimen and ensure she can work towards walking up the ladder by yourself. Fully kitted I imagine moving from a single to double HP80s addeds ~35lb of net weight.
It's not like she wants them throwing rose petals at her feet, carrying her tanks from her car and setting her gear up for her. Some folks just can't ever physically walk two tanks up a ladder - knee problems is just one of the things. I'm sure there's more than a few divers that have physical limitations - my wife is one of them.
You work towards a goal of self sufficiency but sometimes there is a physical limitation. I suppose you'd tell the paralyzed vet that he needs to spend more time in the gym....