I used the all shortening (w/ meringue powder) for an outdoor wedding reception in July a couple of years ago. It held up really well--but I wouldn't try that w/ the BC recipes that call for butter. The ones I've made w/ butter in them just would not have been able to stand up to the heat.
(My mints for the wedding didn't hold up so well. I tried them as an experiment --being somewhat new, and trying to learn--but they slowly turned into little heart-shaped puddles.)
For my recipe, not over 74 degrees at the very hottest, which is basically a warm room. For your future reference, I have heard Indydebi's bc recipe holds up very well outdoors in the heat. If she does put it outside, I hope it's in a shady spot and not too hot. I would make her sign something saying you won't be responsible for any damage to the cake after it's set up. This sounds like a disaster waiting to happen.
Try indydebi's buttercream. It doesn't use butter, but I've never had anyone turn their nose up at the taste and it does get eaten. Did an outdoor wedding a couple of weeks ago and we've been in the 90's for quite some time. I didn't stay for the wedding, but I know that the cake was outside for hours and the leftovers went home with icing intact! 
Did my daughter's wedding cake several years ago - at the beach in NC during August, early evening wedding. The BC frosting held up well. The cake was in the shade and it wasn't brought out until about dark but it was still hot, even then! My recipe uses shortening not butter. Everyone raved about how good the frosting was!!
Good luck!
Leslie
Once I switched to CK Hi-ratio shortening, my BC which does have butter and meringue in it can withstand up to 85 degrees without melting or borders falling off. When I used Crisco, anything over 72 was pushing it!
I was wondering this same thing. I have a woman who wants a "reception" cake (wedding was a few months ago) outdoors in Aug. It gets quite humid here in Summer... I advised her about BC in outdoor heat (no butter in it), but she didn't seem to care. 
I'm trying to persuade her to go with fondant...
Has anyone ever used Sugarshack's recipe outdoors / high heat?
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