Well, its been done successfully before, but no its not the ideal location. There are possible over heating problems, sealing all those wires so the box stays sealed, it would be a maintenance nightmare, etc etc. The amp shouldn't take up much room, surely there is a better location than inside the box. Id even mount it to the box (on the outside) before I attempted this sort of thing. That sort of install is usually only done successfully by sophisticated installers doing a very tight-closed system with little room, or unsuccessfully by a noob who doesn't really understand the problems it will lead to. Which one of those fits your friend?
It would be a headache at best...