First, I haven’t made Rose’s honey buttercream, so I can’t help you there.
You could try using a solid honey like YS’s raw honey. It’s a completely opaque yellow, and it’s solid at room temp. Melt the honey gently—it doesn’t take much heat at all—you can put the container of honey into a pan of warm water (and you don’t want to cook it or get it at all hot). Then let it cool—before it gets to room temp (when it will solidify again), beat it with the butter and any other flavors you want to use (unsweetened chocolate, cocoa powder, lemon zest, vanilla, whatever). It will be soft, but it will firm up with time, because as the honey completey cools, it will become solid. You can refrigerate it to help it along, but be sure to pipe it at the right firmness. As long as you’re in a normally cool place, it will stay as firm as any frosting. If it’s warm, though, it could soften because the honey will liquify—it’s solid at room temp, but not at warm temps.
Although I think this should be firm enough, if you need to firm it further, you can add cocoa butter to the frosting—again, melted, cooled and beaten in. It’s solid at room temp—super solid, like chocolate—so it will firm up anything it’s added to (after it has time to set and return it it’s super solid state). It will add richness, but not much flavor and no sweetness.
Too bad they want ruffles. It seems to me that ruffles would be particularly fussy about remaining stable, as compared to, say, flowers.
Good luck, whichever route you go!
Here’s a link to the honey—you can use organic or not organic:
Not organic: http://www.amazon.com/Y-S-Eco-Bee-Farms-Honey/dp/B000Z93FQC/ref=sr_1_1?ie=UTF8&qid=1397077174&sr=8-1&keywords=ys+honey