This is a horrible decision on the part of your friend unless he has done a lot of homework on the matter.
1. Ventilation jumps up as the first thing that must be considered. (forgetting all else). While he likely can find a projector that works, if the box is built to tight around the projector and doesn't allow for proper airflow into and out of the projector then EVERY projector purchased will overheat and die - very quickly.
2. Projectors, mounted up high, are typically designed to be hung upside down. So, there is not a good reason for a box unless it is a fully ventilated hush-box. More diffiult than some people think.
3. The throw range - yes, you already have that figured out. But, the reality is that if his box is about 12-14 feet from the wall then he likely will be in the 100" screen range size with most projectors. This is not of serious concern to me really if his distance is pretty close to that.
4. The lens offset is a bigger issue. He may have to pony up some extra cash to get the projector to sit inside the box correctly and hit the screen correctly.
I just don't get the 'box'. If you want to hide your projector then get a long throw lens and put it in a different room. If that doesn't work, then go with a projector lift and completely hide it from view. Only a select few people are up to what it takes to properly build a hush box... and I'm not convinced by what you have said that your friend is that person.
Errr.... good luck to him though.
