Your friend will need,
1. An AVR that has preamp outputs for the surround channels. Unfortunately most AVR's have pre-outs only for the front channels, if they have them at all. Flagship AVR's will have them as do preamp/processors.
2. A stereo wireless transmitter and receiver such as those sold by Audioengine.
3. Powered speakers such as those sold by Audioengine. He will need power for the speakers and, unfortunately, a wire connects both speakers (since the amp is located in only one of the pair). Alternatively, use passive speakers and a stereo amp.
Battery-powered speakers are probably not a practical option. Recharging them would be a hassle.