another-view is right - it's important to be realistic with how far they want the signal to go. It'll be more of a problem the further it is, especially if it's an older property with thick walls.
You may not have had to change the channel when you had it, it just depends on what channels are being used around you. If there weren't many people with wifi using the same channel near you, it doesn't mean there won't be near them. He should be able to change the wireless channel by going into the router's menu. This should be accessible via his web browser by typing in:
http://192.168.0.1 or
http://192.168.1.1
Unless you changed the default access codes when you had it, the default username is admin and the default password is password. This will allow him to access "wireless settings" and then change the channel the router is operating on from 1 through 13, with 1, 6 and 13 being the best choices to avoid overlapping (default is probably 6).