she would be better off asking her doctor and PT people. folding chairs by their very nature are flimsy until you get into the high end. If she hasn't had the surgery yet she could go to Bass Pro or other sporting goods store and try them out? a good sturdy kitchen chair with arms? I saw a good number of them at Habitat ReStore last week about 5 bucks each.
I wish her a speedy recovery, do the PT and she will be running soon!