You mean so that you can access his network, or to get rid of the cable?
First, wireless isn't as fast as wired, despite all the ads. In my experience, you lose anything between 20% and 50% of the network speed that way.
Second, an "open" wireless network in a populated area invites the locals to use your network. If they use it for something illegal, you're the one who will get the knock on the door.
You need at a minimum to use WEP, and preferably WPA. If he's out in a farmhouse miles from anyone else, then an open network isn't going to be a problem.
If you have a telephone-type (ie not cable) router and his ADSL is also telephone, it should work (except with AOL which is always different). Ditto cable and cable, but not mixed.