FortiOS Upgrade - Broken HA Pair

Have a look at this : https://community.fortinet.com/t5/FortiGate/Technical-Tip-Unable-to-upgrade-firmware-image-is-not-valid/ta-p/229006

Based on your description I'm worrying about possible hardware/cabling issues with 'A'-unit. First, you definitely need to have console access to see what's happening. We always have a terminal server at all our DCs where we operate an HA cluster and have remote console access to both units to troubleshoot like this. If you don't, you need to get a remote hand with a laptop connected to the console port, then let the tech to connect the laptop to the internet over wifi so that you can remotely access the tech's laptop. So that the tech doesn't have to be so knowledgeable.

But once you ruled or figured out any hardware issues, you need a machine running a TFTP server software so that the A-unit can download a freshly downloaded image from the machine. That requires some skills if you ask an onsite person. To operate the download after flashing the boot drive on the unit, you need to have a console access as well. 
 
If you don't have the environment to remotely operate, you might prefer driving 2-3 hours instead of trusting the onsite tech even if they don't deny your request.

Toshi