I made a barrel cake that sat out for at least 8 hours.  When they went to move it it fell apart.  I wasn't there to see how it fell apart but it sounds like it melted.  I used the blue ribbon chocolate cake and frosting with ganache on the outside then fondant.  It was about 10" tall but I put a support board in the middle with four point support and there was 4 support dowels in the top half as well.  Did it simply sit out to long or did I use the wrong things for this type of cake?

It's really hard to say - especially without pictures.  It possibly was just too warm and the ganache melted causing the fondant to soften too much.  It's also possible there was not enough support u sed.  You don't have any clue how it was handled either.  These are all things that could have contributed to it designating.

Kakeladi is right, if we had a picture it would make it easier to see what collapsed and wher ethe weak point was. Did they send you a picture?

When I do a double-barrel cake, I put a central dowel that goes through all layers including the centre cake board which separates the two tiers.  The bottom tier needs the extra 4 (or more) support dowels, the top tier does not (unless its supporting a heavy topper which mine did) .  I use a hollow poly dowel for my central support dowel - and yes, it takes ages to cut a hole through the hidden inside cake board.

How wide was your cake?  You mentioned 10" tall, but not its width.

Without seeing them 'move' the cake too, you don't know at what angle they had it, whether they rough handled it.  It may be their error and not yours.

Let's see if posting this pic will work.....Here is my 'plan' for an 8" wide and 8" tall double barrel.  It had a heavy fresh floral arrangement on top, thus the extra dowels in the top tier.  The hidden centre cake board was 7" wide.  The centre support was a poly channel dowel, the outer supports were bubble tea straws.

OK, that pic didnt work.  I've added it instead to the gallery

http://www.cakecentral.com/gallery/i/3339686/double-barrel-cake-support-structure