If he wants to go all out, he can go a fair bit further than that. My RS4 is running 18x9+18 all round, the rears sit pretty well like that, but I'm planning on spacing out the front 15 or 20mm to get them flush.
I believe RS-V has 50mm extra track at the rear compared to the RS4, so can go either wider wheels, or lower offset at the rear than the RS4 can.