Indeed. They really need to see their doctor again.

If your friend has already been diagnosed as having bone density problems, I would imagine the doctor has already tested for Vit D and Calcium and Parathyroid or been referred to Endocrinologist?

Something like Vitamin D levels should be checked by blood test particularly if there are already physical changes. It's not always safe to just start taking high amounts of Vitamin D in the hope it'll help. I sometimes see people self-medicating on very large amounts that can in fact be harmful if done without knowledge and/or supervision.

Also, we should never underestimate possible side effects from psychotropic medications.

Regarding the Covid... do you think it's possible they actually had Covid but were tested at the wrong time so it came up negative? Any virus has the potential to cause longer term problems. My daughter, 20 years later, still deals with Post Viral Syndrome (Chronic Fatigue) after having Epstein Barr Virus. She'll be fine for months and then when she gets over stressed, bam, it hits. Never ending story.

Let us know how your friend gets on.
take care