Blind double dates are tough... But if you want to help your wingman, you will have to put up with whatever walks out the door. Keep it sociable and fun, entertain the whole group and DHV your friend in subtle ways. Even if you are not attracted to your blind date, try to keep her busy and give your wingman the time and space he needs with his target.
On a personal note, it happened to me too about a year ago, when I dropped out of gaming a target I liked just to help a friend with his double date. My blind date turned out to be 5'2", while I am 6'4"... Not quite the size I was looking for...

but it was just another day in the life of a dedicated wingman...