How long can Buttercream made with whipping cream be left out? Customer purchased a cake but didn't tell me it would be about 2-3 days before they planned on eating it. They are in an RV and the cakes are setting out. They do have AC, but I could see a sheen on the fondant lettering which leads me to believe it may have not been to cool inside. Also one of the cakes has chocolate ganache well. Any worries?

For the most part the amount of sugar is  high enough to protect the cream from spoiling but.........since you were not informed of the potential problem I'd say you can't be held responsible.  Most of us feel once the cake leaves our hands we are no longer responsible for any mishap.

OR are you meaning the cake was entirely iced w/whipped cream??  That's a different story - as it should only be out for like 4 hrs.

No, cream in the buttercream.

The buttercream should be fine.  Not sure about the ganache, tho.

2 or 3 days sounds about right