There are 100 different ways to turbo a subaru engine.
If he plans on running more than 6 psi of boost he should be looking into a WRX or STI engine swap. The non-turbo engines do not hold up to boost well.
If 6 psi is all he wants he'll need a turbo exhaust manifold and up pipe, a FPR, a better fuel pump, a WRX front crossmember (or a turbo legacy crossmember), an intercooler, a turbo (I'd recommend a TD04 for low boost on the 2.2), an exhaust that will fit, the applicable intake piping to make it all fit, and probably a bunch of other parts. You can do it for around $1500, but don't expect 300 hp without an engine swap.