“Evil” network SSID appearing when router is turned on

I was on a friends network today and we noticed an open SSID “Evil” within range.

This network was on the same channel and signal strength as his. When we turned off the router it disappeared, powered it back on and it reappeared.

I swapped out the router with a spare one and it seem to solve the issue.

What sort of attack would cause this? How can he prevent this in the future?

Any help would be great!