Hi5? Are you serious? I dont see much value in this social networking site as far as marketing goes ... Not when the whole world knows what MySpace can do ... Anyway, its always good to try out new stuff so wish you luck!
Actually, My niche market is on there. Some are on myspace as well but most of my niche market is on hi5.
It depends upon the website he is thinking to market too. Social networking sites are good, but results may not be so productive. I would say try Google Adwords or Microsoft Ad center.