Failed to validate boot image

[wolf_walker]

I've got a Nexus 6 I gave a chick friend a year or two ago she brought back today that will not boot, it has a "failed to validate boot image: ret=-1" error in the bootloader log.
I can't get it to anything beyond the screen with Start in the center and a list at the bottom
of
AP Fastboot Flash Mode (secure)
BL: moto-spq8084-72.04(*) (sha-b11eacf, 2017-07-13 16:59:20
Baseband:
Product/Variant: shamu XT1103 32gb P3A0
Serial Number: (omitted)
CPU etc
eMMC etc
dram etc
Console [NULL]: null
Battery OK
Device is UNLOCKED. Status Code: 3

Connect USB Data Cable

I'm not an android guru, but I did have this unlocked and rooted and a 3rd party ROM on it last I saw it, but I don't remember what. Trying to flash recovery or a new rom don't seem to work and I'm beyond my skill level. Advice?

 

[SactoKingsFan]

You probably need to flash the stock recovery then the latest full OTA firmware from Google. If that doesn't work you could try NRT (Nexus Root Toolkit). It's been known to restore Nexus devices in certain situations.

 

[wolf_walker]

I've used NRTK with no issues on this phone and others before, no luck on this one. All sorts of weird errors and timeouts, not a driver issue I don't believe. I've had it list the bootloader as unlocked, than attempt to flash full recovery and stock image and it'll then say bootloader is locked. I'm starting to think something hardware wise has died.