No power after faulty USB hub plugged in - iMac 27 Late 2013

A customer brought me an iMac 27” Late 2013 for repair.

Issue: After plugging in a faulty USB hub the iMac went dark. Since then the customer was not able to power it on again —> No power - no fans spinning, no chime

What has been done so far:
I have opened up the iMac and checked the LEDs —> none of them light up, if i plug in the Mac and press the power button.

I assume it is a fried logic board, but is there something else, I could check, besides taking out the logicboard and check for burned areas?

Thanks in advance!