I'd approach it like this, I'd say "look we will both benefit, A. I will look better and all of your friends will be so jealous... it'll make you feel and seem so accomplished" B. Go ahead and throw in the "girth line", it can't hurt... C. offer to do so IM injects in her glutes prior to the wedding and explain to her how the pump will make her booty look so good in that brand new dress... plus it works in your favor at the end of the night.
If all else fails start returning wedding gifts to make up for the financial commitment youve made to the greatest supplement ever... MC IGF. the pump is out of this world... and remember what Arny said about pump... yep... either way your good!