Replace the motherboard if the PSU and CPU were both damaged.
The PSU provides +12V to the CPU VRMs on the motherboard which in turn down-regulates that to the low voltages needed by the CPU. If the PSU went bad taking the CPU in the process it HAD to damage/overstress at least one of the VRMs too, the same is true if the CPU went bad taking the PSU with it. The VRM can neither be replaced nor economically repaired so just replace the motherboard to be certain.
The VRM may function for a while, but the devices (FET's) are almost certainly over-stressed and weakened. There are no tests you could do to find an over-stressed FET that might fail when pushed hard with a new (and doubtless more power hungry) CPU. You're very lucky testing your friend's CPU/GPU in that board since the weakened FET's could have failed and taken his CPU. I hope you used a new PSU, at least.
And I'd replace the power strip too. The surge protection devices in even the best power strips have a limited number of 'surges' they can absorb before they have been exhausted. When they fail they do not give any notification of it and the power strip continues to work normally. There may be some at the very high end that do give a notification but they will most likely also have replaceable surge protection devices and you'd know that when you bought it.