Question [Q] Stuck on boot after KK update

This is for my friend's ATT One. Bone stock, unrooted, S-On, no changes made to the device from the store. He took the OTA for Kit Kat and was running fine, then his phone rebooted on him a couple days later and won't go past the white HTC boot screen.

Two problems: First, I cannot use ADB, as it does not detect his phone. When I plug in his phone, it says drivers failed to install for HTC MTP device. I also have an HTC One, unlocked, S-Off, etc., and when I plug mine in, it's detected and ADB works just fine like always (so it's not that I need to reinstall drivers).
Problem 2: the phone seems to hang trying to do a factory reset from the bootloader menu, or from the stock recovery menu. It will start the reset, reboot to the white HTC screen then hang. I left it for more than an hour twice. Nothing doing.

Is there a way to reflash/restore his phone without wiping his sdcard? He's OK losing the data partition, but wiping sdcard is last resort. Using an RUU .exe will wipe his sdcard, as will unlocking his bootloader to try to install a custom recovery. Just not really sure what tools I have at my disposal with a stock recovery and no adb. Will fastboot flashing a rom.zip extracted from the ATT RUU also wipe his sdcard? Or does that only happen with the RUU .exe? Any ideas appreciated..