I dunno, I dated a girl for a while that was this dumb. My friends and I would make up words and inject them into conversations around her. Then we would take bets on how long before she used the made up words in conversations with other people. It was hilarious to look at the confusion on their faces. It did not last long, I couldn't take the embarrassment of being in public with her. Then again my wife is much smarter/wittier than me, which is a whole other set of problems.
Invite them both over and think of some "dumb tests" to try on her. It may turn out to be amusing. Give your friend another 6 pack and throw a couple extras in for having to deal with her being an idiot. He could use them.