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