Which coolant for high altitude in winter?

High altitude makes no difference. Read the chart on the bottle and use the correct antifreeze to water ratio. 50/50 is the normal mix which protects down to minus 34 degrees F. You may need a cooling system service or there may be another problem such as a leak or blown head gasket.

Did she check her fluids before she left for her trip? If the coolant was low it may have been fine on flat roads and other normal driving. But driving up mountains means the engine is working harder and generating more heat. Is she pulling a trailer and/or have the vehicle loaded with equipment and extra passengers? Again the engine will be working harder. Wait until the engine cools off completely and then fill up the cooling system with your 50/50 mixture of antifreeze and water. You can buy coolant already mixed so you can just pour it right in. I'm betting she was low on coolant to begin with. Hope that helps! Jim