wow! do they make ovens that big? lol. seriously though, i have never seen pans that big, but this is what i would do... make 4 12" square cakes, put them all together to make a 24" square and carve them out into a 24" circle. to carve out the circle you can measure a piece of string 12" long and tie one end to a dow or something, stick it in the middles and attatch something ...or even a knife... to the other end so you can make a perfect circle. you will have lots of leftover cake but just charge her extra for that, lol. ...there might be a better way to do it, but it's an idea 
Good idea. Too bad it got it's own thread instead of a reply to her ? 
http://www.cakecentral.com/cake-decorating-ftopicp-6392918-.html#6392918
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