Re: I once had to do something similar in a Skoda...
I once had a similar experience in sunny New Mexico in the late 80s or early 90s - a somewhat older vehicle was standing by the side of the highway showing clear signs of overheating. We stopped and offered assistance and showed them the trick of running the heater to help cool the engine.
They drove off happily, and we followed for a bit to make sure all was well. Sure enough, after a little while, they pulled over again. Turned out it was too hot in the car with the heater running (it was nearly 40 degrees outside), so they'd switched on the A/C, and a late 70s/early 80s A/C compressor represents a significant engine load...
Advised them to roll down the windows instead, which got them to the next exit.