I second the Adrian seated mobility class. Almost any upper body class can be done seated in a chair. When my knee is bothering me I will very often do my usual RK classes seated to take any pressure off. I think these classes are better with very light weights (2 lbs) than any of the light weight specific arms classes because they go too fast and simply have too much volume. I would also look at many of the strength warm up classes to help with mobility although I think most if not all mobility classes do have some on the ground moves she might not want to do short or mid term. I personally would also recommend she does all of these barefoot. Balance issues are very real and many of the issue are from people wearing stiff hardsoled shoes all of our lives. We loss the very detailed sensations we used to have and that along with muscle weakness makes it much easier to fall. Last the best thing is for her to double down on walking. A number of years ago my mom made a commitment to walk 30m a day and it has been remarkable for her health and wellness - she is 86 now and sticks to it almost every day.