They can't outright void a warranty due to mods (Google for 'Magnussen Moss act' if you want to know more) but with it having power mods, most dealers would make it really difficult.
It may not matter anyway though since the 5year/60000mile powertrain warranty is all that's left and depending on when it was originally purchased, that could be up in a matter of months. I think the 02's were here as early as April of 2001? Memory a little hazy on that. At most you don't even have a year left unless he purchased a transferable extended warranty.
Make sure the tranny is in good shape. Subaru didn't put in the clutch delay valve until the 03 models, so aggresive launching and/or bad shifting can put a ton of stress on the gears.