6x RX 580s (185W x 6) is an overload for a 1,200W PSU.
Even though that model has 100A on its +12v rail, it is still at its peak with a 1110W load (only have 90W window).
Since you're mining Ethereum, it's alright to underclock to lower the power consumption, or use only 5 GPUs for now.

And this bugs me: "my friend has two rigs with 2 x 6 RX 580 sapphire nitro 8gb"
Two rigs with 2x6 RX580, means 12GPU in each rig? No, right? so 6GPUs each.

The error is pretty normal and shouldn't cause a reboot,
both of the rigs seems out of juice that can trigger a shutdown but your BIOS was set to restart whenever the system was abruptly powered down.