I think he needs to stick with 225 tires due to the 'wrong' offset. For an 18x8.0 wheel, ET38 will pull it inside another 7mm (1/4 inch) for fender clearance. Any more than ET38 and it will probably rub the strut tube. And anything wider than 225 will probably rub badly on the fender lip unless you install 500E fenders, skirts, and bumper. I'd almost say swap the wheels with something that lets you maximize tire size (i.e., 8.0 ET38 or ET40), but I suspect he wants to keep those AMG wheels.
I also wouldn't run staggered tire size on the same size wheels, I'd put 235/40/18 front & rear - plus that way you can rotate them. Now if you stuff 8.5's in the rear, that's a different story! :p Having 18's lets you squeak in a few more mm of offset (ET40, 41, maybe?) where a 17x8.0 wheel is maxed out at ET38... the inward angle of the strut tube is what lets the larger diameter wheel tolerate a smidge more offset. Hope I'm making sense here.
On a side note, a 245 is about the widest tire I'd want to run on a 8.0 wheel, anything more and it will "swim" around and make the handling sloppy, plus it will look 'wrong. But in a straight line (braking/accel) there might be gains. Optimum is 245 on 8.5, 235 for an 8.0, 225 for 7.5, 205/215 on 7.0... YMMV, etc, etc...