DD has one hand in a cast and one of the few things she does is put her feet onto furniture sliders and use her forearms to pull her body along the floor. I'm not sure how much that helps but it is something.
Also there are many exercises with therabands that she could probably do, at least with one arm. If it's just her hand in the cast, she could put one end of the band around her forearm and step on the other end and do some arm curls, for example.
I think it depends on what her weight bearing restrictions are with her cast. When DD got her last cast, she was full weight bearing with it on. So she really did everything she could- even some push ups with it on. Crazy. However, when it came off, she was weak. Like really really weak. That plus the loss of wrist flexibility was tough on her. Wrist flex took around 4 weeks to come back 100% (she was immobilized for a total of 6 in her cast) and strength took around 5 good weeks to get "normal ish". That was with strength work at home, an hour five days a week with a PT/ trainer, plus full gym hours. The thing that took the longest was bars. She worked seriously hard on strength (still is working with the trainer) and it was a rough time there for a while. DD was really discouraged and shed a lot of tears. I want to tell you this because I don't think I was prepared for just how weak she would be when it came off. It shocked me- I mean I had broken bones as a kid, and don't recall this kind of weakness, but I didn't walk on my hands either! So, my advice is to just be prepared when that cast comes off- it is going to be rough for a while. But she will get through it.