www.raspberrypi.org - Raspberry Pi is a credit card sized computer, everything in it. Operating system is installed on a SD card. The luxury model is about $35. They used to have A and B models, A is the cheaper ($25), but it doesn't have so many USB-ports and no network port. I have the original B model and it runs xbmc quite ok. Then they made A+ and B+ with more power, more USB-ports and now the latest is Raspberry Pi 2 with even better specs than B+. That came out very recently. And the price remains the same, while the hardware gets better and better. Mine was Velcroe'd on the back of my TV with the IR-cable coming up on the top of the TV. Almost invisible. (Nowadays I have a miniPC for Kodi and the RPi is a baby cam.)
I think they scrapped the A model, because there is only one type of RPi 2 and it looks to be the B model line. So now it has quadcore and more ram. I had trouble running HD-video with the old B model running Frodo, but I think the new RPi2 should run it ok. At least the specs look like it can.
It has HDMI output as well. And it is easy to build an IR-receiver for it so you can use any old remote control for it. There's plenty of how to guides on Youtube. Or don't build an IR-receiver and just use Yatse or some other Kodi remote on your Android phone as remote control.
If he is looking for a cheap client and has all the media on a server, I would recommend Raspberry Pi 2 as well. OpenELEC just released a new version of their operating system that has the new Kodi 14.1 on it. Easy to install and they have easy to follow guides how to do it.
Of course there are other similar computers nowadays, like Odroid, but they don't have that much of community support.
3 PRi2 models would cost about $105 for the boards, and whatever you pay for a micro SD card for the operating system. Small 4GB card or whatever you find is enough. And operating systems with in built Kodi are free to download from the net. I guess he already has network cables to connect them to the network. You only need keyboard and mouse during the installation and after that just use remote controls (I use Yatse for Android). For the power he could get either standard micro-USB chargers for $5 a piece from Amazon or if the TV has USB-connectors you could try to plug the RPi2 to the TV with a USB-to-microUSB cable for the power.
