Fundamentally the policy is that everyone should be sat in the assigned seat. There are some harmless exceptions made, like when willing parties swap with each other.
It's a little more delicate figuring out how to gracefully invite enforcement of the policy, however. Good FA's are vigilant, and I have seen them spot / confront poachers, but not all do so.
You perhaps could have asked the possible poacher simply if the aisle in your row was her assigned seat, and then gauged the response to decide if you want to call the FA again.