One question I wanted to ask is: How much training did this mare have before she went lame? Was she a seasoned riding horse or was she green broke? Has she been ridden before? Oh, I guess that was 3 questions.
In all honesty, I have to agree with Skips. She isn't your horse. Unless you are being paid to fix her issues, then just exercise her on the lunge. There is no sense in risking your health or hers unless that is what you signed up to do.
Lily used to be a show jumper, but for the past few years has just been used for breeding, and general riding. Her owner at the moment is actually a novice, and she's never had a problem with her. :)