My middle daughter is lactose intolerant too, and there ARE calcium supplements that are lactose free. Your mother can also find calcium fortified foods that don't have milk in them, such as breads by Bimbo. Kraft and Cabot brands make many cheeses that state right on the nutrition information "O grams of lactose" so she can get some calcium that way.
I just recently read that CVS brand calcium supplements are soy, corn, gluten, dairy, wheat free so I'll be checking those out in the next few days for my daughter. Her nutritionist wants her to have 800 milligrams per day, which she can get from various sources like Silk soymilk and the other fortified items I mentioned. So we're supposed to supplement her with 500 mg if we can get her a lactose-free one.
I also read that Whole Foods has a good selection of vitamins (why I never thought to check there, I'll never know) and that they have a lactose-free calcium supplement too.
Personally, I would try to get her to take the supplements, continue with therapy, and ask the doctor about medications like Fosomax (maybe not that exact one, but other anti-osteoporosis meds, especially ones with a lower risk of causing blood clots). I've seen some of my friends' moms get back surgery and a woman who used to be a neighbor, and while it worked for two of them, my neighbor wishes she never had it done. Another friend had one done and she's good some days, but horrible others. I don't believe any of them are in your mother's age range or in as poor health. Back surgery at your mother's age would require very, very difficult recovery and it would be long and very painful. She needs to know the chances of success in general with this surgery, the statistics of this doctor performing this surgery and his rate of success... she needs a second and third opinion as well.