Okay, I understand what he's going for, because it is really popular now to save money on the cake by buying a sheet cake... however, I think he may be doing it a little wrong. Normally people will buy a 3-tiered cake for the bridal party and then sheet cakes to feed everyone else. It may look a little tacky to include the sheet cake into the tiers, but if they don't care about the way it looks, then who am I to say anything?
If they wanted the tiered cake for display I would suggest not including the sheet cake in it. That should be kept separate and used only for the serving of the guests.
Ultimately, it's their day, and if they aren't trying to impress anyone then it doesn't matter. If they are doing something pretty formal, then I would say its tacky.
