We have witnessed the results of rebuilding grand pianos after being expose to heat from a fire. In due course glue joints not addressed in the rebuilding commenced failing. Unknown was how long the instruments were exposed and the intensity of heat. While these few incidents do not supply enough information to draw hard conclusions about a piano being exposed to 120+ degrees for an hour or more, in the absence of studies concerning the long term effects of this level of prolonged temperature upon pianos - at the very least, lethal to humans - perhaps removal to a safe place might be the wise course.
Let's come back down to earth here. 120ÂºF is not "lethal to humans". I've been in those temps at the bottom of the Grand Canyon. (Not minimizing the opportunity for death by dehydration or heat stroke but not intrinsically fatal)
For soundboard repair, it is not uncommon to dry the board (i.e. including the whole rim) out at those kinds of temperatures. My previous rebuilding shop was in a former grocery store. The meat cooler was well insulated and served well for drying piano soundboards for repair as well as for maintaining raw soundboard panels. We dried boards for days in order to reach minimum moisture content before shimming. In an hour, the piano would be starting to get warm. Fires are much hotter than 120ÂºF. I've seen phones on the wall melted into a Salvador Dali-esque shape. It gets really
hot in a fire. The big thing that kills glue joints in a fire is not just the heat, but the steam that is generated when the fire is put out.
It would probably be fine to wrap the piano in several blankets or that aluminized bubble-wrap type insulation. Nonetheless, I still concur with others and my original statement that it would be safest to remove the piano. At least, any unwanted results would be at the owner's risk.