That red/black rashguard would look great if it's the right red. I have two s/s zippered ones like that and I simply love them; they keep my ginger self from getting burned and I can unzip them to cool down a bit when out of the water. If you can, do avoid buying a black rash guard; they get very hot.
A white top may look really cut with that skirt, if you can find a nice one, and if she tans, there is nothing like a white top to show that off.
One thing to remember, since she's well-endowed: rash guards have no support at all, and she'll need it. Go ahead and buy her a plain tankini top with good support to wear under that. (This is where your daughter will be needed; she'll know what her mom hates or loves in terms of bra fits.) Venus has some nice plain tops, so you might look at those, too:
https://www.venus.com/swimwear.aspx Personally, I'd advise against any kind of ruffily-hemmed top with that skirt; too much fluff around the middle, and no woman likes that idea.