Is it plausible that an interrupted Windows update can cause the motherboard to fail?

(The answers given by others are, I think, wrong).

Although somewhat unlikely this is possible. Manufacturers can indeed get Windows to update firmware and UEFI during a Windows update, and if this process is interrupted you could indeed hose a motherboard.

See https://docs.microsoft.com/en-us/windows-hardware/drivers/install/updating-device-firmware-using-windows-update to read it "from the horses mouth"

The most common way of updating motherboard firmware is known as "UEFI Capsule". This method is supported by Windows update, fwupd and many manufacturer applications.

UEFI Capsule firmware updates work by copying the new version of the firmware into a specified location on the EFI partition. Applying this update is NOT immediate, as UEFI becomes read-only when control is handed over to an OS. Instead, upon each boot UEFI verifies if there is an update Capsule in the specified location, validates it's signature(which would have prevented the update from being applied if the file was corrupt), and applies the update.

Based on the UEFI specification, corrupting the motherboard firmware from a running OS shouldn't be possible. However, if your motherboard is old enough to use BIOS instead, it might be possible.

I would say no, that a Windows Update will not break a motherboard. However if you had a power outage (not entirely clear), then the surge from a power failure could indeed damage the motherboard. I think that it was doing Windows Update was incidental to the damage caused by power failure.