I don't think I'd have been too bothered by your ex roomies behavior.

Leaving that film is how the seasoning gets improved over time. Nor are cast iron skillets expected to remain pristine the way they came from the store.

Long as there wasn't a sign of food stuck to the pan, or the remaining film was not sticky or easily transferred to other places, it's fine.

Skillets like this are for frying food, they get hot enough to sterilize whatever you might worry about.