I sleep on my side (they say it's bad to sleep on your back because the weight of the uterus and baby can block bloodflow to the placenta) I use a pillow to rest my knee on and kind of put the pillow in between my legs to help.
Also if I want to rest my back a little while I am sleeping on my side I put a pillow up against my back so that I can lean back and rest my back without having to lay flat on my back.
Or... She can always get a pregnancy pillow to help her out. There's different makes and different sizes, it would depend on what would work for her.