Where is the switch behind the glove box that switches between air conditioner & heater?

There is no switch that I know of. Your friend may have disconnected the heater core bypass door. See if there is an electrical connector that was disconnected or some sort of mechanical linkage. Why was the modification made?

Because my a/c stopped blowing cold air and blew air hotter than it was outside.

There is some sort of switch my husband does. This every year. I remeber him saying the air has ti be on for it to work. Its a pain each year. And every time he figures it out he says boy that would have been easy if i could remeber where it was. It takes him about 1\2 hour or so to find it and Get it to work. And because his hands are so big the get cut up by moving around in that small space.