It's the light, not the heat. Look at any poster taped up in a store-front window. They turn from full color to cyan and black in weeks--in freezing cold weather, just from the sunshine.
I say that if the lady wants a slide show with her valuable old Kodachromes, keep it short. No need to project it for more than 5 seconds. Scan it, dupe it, or whatever to print it. But protect it.