I have to agree with most of these comments. Fake costs just as much as real cakes so I don’t see how its going to save her money unless you are undercharging for the dummy cake! But to answer the question, I surely do not think it is tacky to have a buttercream sheet cake and a fondant display cake. I doubt the guests would notice there is no fondant like others have said!
-- Nikki, So Cal, www.Facebook.com/nikkibelleperchecakes