I'd say Wireless is the way to go, if rather than a new modem (And new account with new monthly fee, a reccurring cost) they got a pair of wireless access points, from the same manufacturer to ensure absolute compatibility and set one up as a repeater - a feaure on some access points. e.g.
http://www.dlink.com/products/?pid=267
Or settle for just one access point and high gain antennas, if they really want to go to extremes along that route a
55-mile Wi-Fi connection is possible. (The other furthest one was to a weather balloon!)
The powerline based networking solutions have a reputation for being extremely unfriendly to HAM radio users and shortwave listeners so may be worth avoiding.
Regards
Memetic