Do you have a nandroid of any 4.4 ROMS saved anywhere? I was going to say try and flash the stock image. After it gets through flash and tries to boot into recovery, go back directly to download mode. Flash philz or twrp, whatever you need to. Try and perform a recovery boot, wipe everything, and nandroid restore of a 4.4 if you have one.
Last resort try the sd-card boot. I don't necessarily think it will work with this particular case as you do not have the dreaded QHUSB_MODE and your phone is still recognized by the computer(s) you have been using.
But you are right -- he should try everything else before the SD-card boot. Because if something else succeeds then maybe the hardware is fine, and if he went immediately to SD-card boot he wouldn't know that.