You are right, but your mom would easily benefit from someone else who would be able to fix that: free software relies on the fact that someone "able" is at least not looking to hurt you, which is, for a popular enough software, a given.

For software that is not popular enough, you are right that in practice, your mom wouldn't be able to do anything other than stop using the software.

But of course FOSS is not perfect, it's just a way to empower your users: nothing more, nothing less.