Were the tan areas kind of thinnned and crispy/ papery? Sort of like this:
https://bifurcatedcarrots.eu/wp-content/ ... _scald.jpg
If so, probably not diseased at all. That would be sunscald, which happens when seedlings that have been in some sort of protected situation are abruptly moved in to direct sun. Usually it would not kill them, unless severe and continued. Treatment is to bring them back to some protection, diffused light or dappled shade, keep them well watered and gradually expose them to direct sun, starting with just a couple hours.
The burned leaves never recover, but the new leaves that emerge are OK.
The Home Depot etc seedlings may be fine because they were already "hardened off" used to being in direct sun. Your friend may have had his seedlings indoors, in greenhouse, or otherwise more protected.