Talk to her doc - there are once a week pills for vitamin D (I use these), but they need to be recommended by a doc. Gel capsules, that can be dissolved in something like soup. Talk to the doc as there may be other options.

The osteo pills I use are once a week and a bit of a pain and probably won't work for your mum. I have to get these pills through an endocrinologist too. But there are alternatives, including a six-monthly intravenous infusion that has it's own risks & benefits but doesn't require any kind of pill.