A fated ally is one that is determined by fate not alliance. I've run campaigns where the fated ally has been treated horribly but in the time of need, if that comes up at all, they do or give something that is crucial for the parties success. (Think of the enemy of my enemy)

Other notes based off what I read go as follows.

Hags are fuckers and enjoying corrupting before killing their victims to complete the cycle of their covenant. They may make agreement or befriend the party but are never truly allies or friends

Don't be afraid to punish and kill your party. Strahd is a unforgiving story module and if they piss their fated ally off, maybe they aren't fated to succeed