220 volts where it should be 110V. Why And how to fix?

My neighbor asked to take a look at her light outlet. I do most of my own home repairs including installing new electric to renovated rooms. But this one baffled me. She said that the light went out and so she changed the bulb. BUt it still doesn't work. So, I went over to look at it. Sure enough, a new builb that lighted up on a different screw-in socket did not work on this light bulb socket. So, I broke out the tester to see if there is power to the socket with the switch in the "on" position there and there was power but it reads 220V not the expected 110 or 120v. What's the problem? Could it be switch that controls the light? Or could it be that two blacks from seperate breakers were joined - but that wouldn't be because they didn't do that and the bulb was working on the socket before.