I have a friend that became Habad a while ago, and lives with her non-orthodox parents. She leaves the house for shabbat, but insists that her parents turn the lights off in the fridge, because if they open the fridge with the light, they will un-kosher her food. IS THIS TRUE?! I thought the prohibition was against opening the fridge because she (or they) would be using electricity, but not that it would make HER FOOD unkosher!!! Please help!
If someone cooked some food on shabbos, which transgresses shabbos law, it becomes non-kosher for the person who did the act (this also applies to any other act which transgresses shabbos law but cooking is the most common example). As well, a Jew may not benefit from the transgression until after the shabbos, and some say they have to wait, then, for as long as it would take to cook said food.
However, when it comes to a fridge, the opening/closing doesn't make the food, and only has a slight effect, and probably would not be a problem. There's also the fact that the actual act, i.e. opening/closing the fridge, is not a melachah (shabbos "labor") itself.
As with any halachic question, consult your local rabbi (or tell your friend to consult theirs).
See "Tur" and Shulchan Aruch Harav siman 318, as well as Shulchan Aruch there with the "nosai ceilim".