Is it possible for your mum's GP to refer her to a consultant oncologist for advice? Presumably she is no longer directly under one, it being at least 10 years since her treatments ended. I think there are reasons for not giving hormone therapy indefinitely and that after a certain length of treatment, it ceases to be effective. You need to discuss this with a specialist.
You could ask about bisphosphonate therapy as it is a treatment for osteoporosis, and there is also evidence that it can reduce the likelihood of secondary breast cancer taking hold in the bone.
The websites I have been advised by my oncologist to keep to for information are:
The NHS sites
Breast Cancer Care
Cancer Research UK