Stress testing shouldn't break a modern PC. Back in the olden days it was possible to break GPU's that way but that was a long time ago. Everything has built-in safeguards against over-heating now. The easiest way to push a GPU and CPU to get constant high temperatures is probably by using something like 3D Mark and PC mark. They are after all designed to try to do more than current hardware can handle. If the owner doesn't feel like downloading and installing things I guess your best options are to try a recent game they have installed. Check that the settings are maxed out and just see if the performance deteriorates as it heats up and if the fans are going completely bonkers.
Another super simplistic test is to view a long 4k test video on YouTube in a windowed browser (with the desktop visible beneath the browser). Make sure the video is manually set to 4k when playing. That usually forces the browser to use CPU for rendering as it can't use the GPU's full screen features. This isn't super stressing on a current PC, but, if it does over heat from this it's definitely having some issues.