My best suggestion would be to give her a dose of bute before trying to trim her. That way, it won't hurt her to bear all her weight on that one leg. Plus, give her plenty of rest breaks.
Clean it out, set it down, trim one pass, set it down, trim another pass if needed, set it down, rasp a bit, set it down, rasp a bit more, set it down, do a bit of final shaping, set it down. Plus, any time she feels like she is having trouble or getting tired, set it down. Give her a break of a few minutes between each time that you pick it up.
Those hoof stands are extremely handy but you may have better luck with just a person that is able and willing to hold her foot extremely low while working on it. Another thought is to have one person hold the foot and another person work on it. That way, the holder can use their hands to hold it instead of their knees and keep it as low as the horse needs it to be comfortable.
When working on her foot, try to keep from putting any kind of torque on her foot that would cause her leg to pivot at the knee joint. I've known some farriers that tend to clip a spot with the nippers and then pull/tear at an angle to get it loose and that would cause the horse's leg to move/spin/pivot in their grip. Not that big of a deal for a healthy horse but painful for one with bad knees.