What are minimum voltage requirements?

In trouble shooting some problems at a client site, we measured the voltage output by a UPS when it was unplugged from A/C and the computer was running. The voltage sagged to between 80 and 85 volts. The hard drive was trashed by this test, and ithad to be reformatted. Our client was watching this test but still doesn’t believe the voltage caused the problem.

Does anyone know what the safe limits of low voltage are for a computer, before damage is possible? We need some kind of documentation to show our client.