YSK: If you make popcorn in a pot on the stove, the oil to use is ghee.
Why YSK: Popcorn fans often want a buttery flavor, but plain butter is a bad choice for popping popcorn in a pot, because the proteins and sugars smoke and burn around the same temperature where it's hot enough to pop the kernels.
Ghee, or Indian-style clarified butter, is butter that's been simmered and the milk solids (proteins and sugars) skimmed off. This leaves a clear yellow oil that doesn't smoke when it's heated and doesn't go rancid quickly, but has a distinct toasty butter flavor.
Vegetable oil is either flavorless or faintly bitter, and some high-temperature vegetable oils tend to start polymerizing (i.e. becoming plastic) when heated in small amounts. This is also not good for popcorn.
Good-quality popcorn popped in ghee reliably produces lots of "butterfly" popcorn with few unpopped "duds" and no scorched kernels or batches ruined by smoke.
Although it probably doesn't kill the housemate with the peanut allergy, it makes them very uncomfortable and possibly costs them a significant medical bill.
Should note that refined peanut oil will not cause allergic reactions in the majority of peanut allergy sufferers (although it really isn’t worth risking).