If they haven't got much hiking experience, what's their gear like? Are they used to hiking with it?
When you do a training hike you don't just train your muscles but your use of your kit. So before I did a 2-week hiking loop including the northern 1/4 or 1/3 of the GR20, there were several mandatory full-pack training day-hikes, in the boots we expected to use. By the time we went everyone had also been on at least one two-day hike with a full pack, camping in between.
The approach of wearing in boots (and feet) and getting used to the load can be used to reduce the emphasis on fitness; hiking-fitness is anyway a better way to approach this than weight.
A further point in response to your edits saying she's actually obese. I'd actually worry more about the descents than the ascents, for the sake of her knees (and yours if you end up lightening her load). I picked up a minor knee injury, that nagged me for years afterwards, descending into Calenzana and on to Calvi when I was young and fit (and carrying around 25kg, because one of our group was struggling). Her body weight alone will put a huge load on her knees going down. If we assume for the moment that she trains hard and goes for it, she should probably train for descending with trekking poles. In fact, you all could, which adds an element of "all in it together" on the training hikes.