you can get her to kneel on the couch, obviously not with her heels touching her bum!! so shes in an 'L' shape. then u can wax the bottom part of her legs and as you get towards the backs of her knees and her thighs just get her to lean forward. you could even put the back of the bed up for her to lean on if she cant support herself.
alternatively do the backs of her thighs from the front, just get her to bring one leg in to her chest, wax the back of that, then repeat on the other side.
Or as has already been mantioned, get her on her side.
HTH's
=]