Side effects lists are always scary.

Her doc is likely weighing the chance of those side effects occurring against the risks if she breaks a bone at her age & other conditions—IOW, a numbers game.

I don't want to dismiss the side effects side, but a lot of them are infrequent to rare, and many (if they show up) can be avoided/reversed by notifying her doctor and discontinuing the Prolia.

Common and Rare Side Effects for Prolia subcutaneous

And consider—the list for Fosamax is just as scary.

Common and Rare Side Effects for Fosamax oral

vs.

Facts and Statistics

Hip Fractures Among Older Adults

incidence of bone breaks in elderly women

This side of the equation can be pretty scary too, e.g.:
Doc