There's no way I could pull that off with the weather we have here. I have seen some people suggesting that the cake be iced with buttercream made with all shortening with some butter flavor added, kept in the fridge until the last possible moment, and then taken out and kept in the shade at the reception.
Maybe you could make a practice cake, just one small tier and chill it for a while, then put it outside in the shade and keep track of how long it takes for it to melt. If you try it and it just isn't going to work, you could tell the bride that you can either do cupcakes (which won't look as bad when they melt) or fondant. If all she's had is wilton fondant, you could also try making some mmf and give her a sample of it...flavor it with orange oil or one of the lorann oil flavorings so it tastes like something more than just sweet. I've noticed that mmf will survive a lot better in the heat than buttercream.