Well, I was able to fix his truck, but I have no idea what caused his podding problem. I told him I was going to post this info here in case an MTM2 forum member may know what went wrong.
I first extracted the files from his POD, and tried to open his model in Binedit, but it would not open.
When I opened his POD file using podview once again, I found that every file within it showed zero bytes (see picture)
Fortunately, he had also zipped up his entire art and models folders (and more, as I found duplicates of those folders in his ART folder), plus there were more files in a file called truck.zip.
By searching through all these, I was able to find copies of his custom model and texture files that contained data. I installed them and found I was able to pod up the truck in Tracked2 with no problems (no errors in the TRK file). I tested the truck in the game and it ran error-free.
And so, I have sent the podded truck back to him, but I could not tell him why his pod contained empty files (as seen in my picture). If anyone knows what went wrong, please post here, so he will know what and what not to do when he starts his next truck project.