Hi protectanimals, welcome! What your mother-in-law is experiencing is something we all may deal with at some point in time. Here's an article that will give you tips on what to do when you disagree with your doctor.
Once you've tried this see if it helps. You could also try going with her for moral support, or if you can't go (time or distance problems) see if a friend or other family member can go to help out with the issue you are describing.
Also, the FDA/ASBMR (Amer Soc for Bone and Mineral Research) is warning those on these bisphosphonates (Fosamax etc) that long-term use may not be beneficial for some patients. If she is at high risk for fracture you have to consider doing something, as far as treatment goes, and make a decision to stay on or take a break from the Fosamax. Only your mil and her doctor (together) can do this. It is not wise to take advice from anyone who doesn't know your mil's medical history inside out. You can read the warning on bisphosphonates above, print it and have her take it with her to her next appointment. Be sure she is prepared to explain all of her concerns and have something to back that up like the printed link above, from a reliable medical source. Once all issues have been addressed, see if the doctor has a change of mind, or can explain the significance of staying on Fosamax, taking a holiday or changing to some other treatment altogether.
I hope she can reach an agreeable resolution with this and still treat her bone loss to protect her from fracture. Hip and spinal fractures are very devastating, so keep this in mind as well, even if she decides to change the treatment plan. Also, consider a second opinion if her insurance covers this, or if she can afford it, in case they don't.
Best of luck to you both and if you have another question, don't hesitate to post again. Take Care!