Since he already has the piping (i'm assuming he's still using the stock turbo) then I'd suggest getting an STi turbo. It'll bolt right up (keeping the pipes he's using now) and w/ a vf39 he could put down 350 whp fairly easily. They go for $350 - $500 w/ low mileage making them a fairly cheap upgrade for wrx owners.
Has he upgraded the fuel pump or injectors yet? He'll probably want to think about that before the new turbo.
He shouldn't have to get a new tranny w/ only 350 whp. It all really comes down to how he drives it and if he's upgraded the clutch. I'd def. leave some in the cash reserves just in case the tranny went for whatever reason (bad driving habits
) but it should hold up.