little late to the party here...
i don't recommend ace bandages. they're okay for cosplay, but if you're talking about a kimono that might be worn in different situations, you don't want to worry about slippage. ace bandages can move, and she'd have to go adjust herself several times while wearing it.
i have a compression shirt, and i lose something like 4-5 inches. my chest is rather large, DD-DDD, so i don't get flat, but it certainly helps, and they don't bounce around like they do in a bra (even a sports bra bounces more).
i got it from underworks.com
you can't wear it with an outfit that's low cut in the chest, though, because you'll see the top of the shirt. kimonos cross up pretty high on the chest, so it shouldn't be a problem... but... just a warning.