Well, an easy way for them to check is if she sometimes accesses Netflix (using the same MAC address, such as on a laptop or iPad) at your home as well as at her dorm. Even if there are ten or fifteen MAC addresses that access via the same account at multiple locations, they might only need one to travels back and forth to consider the account valid.
We'll see how this shakes out. I suspect the bigger issue that Netflix is looking for are multiple IP locations used for access, with NO common devices used in both locations.