Without boning that dress will pull down like there's no tomorrow. Also, boning does not mean she will not be able to breathe or dance. A strapless with a heavy skirt means a boned, tight bodice is mandatory. I'm afraid not even clear lingerie straps will be able to support it. A yoke will do.
I had a strapless princess wedding gown. Was able to move and dance, but man did I feel that weight all night.