I dunno, seized sliders will usually just cause a pull and heat up the rotor, not lock it up so solidly that it'll drag without rolling. That's what the OP described. Back in Jersey I saw cables with bad upper seals fill with water and jam from the combination of ice and rust, it doesn't happen a lot but it does happen. Maybe water in the brake fluid too, I could buy that on an older car if it's never had a flush. Could also be mechanical, and the maybe cold is just a coincidence. Without a blowtorch or a warm garage, she may not find out till it thaws.