Favor for my wife

Over wine, my wife and her friends got on the topic of cunnilingus. One friend in particular was completely unsatisfied with her lover. My wife decided to offer me out, but with a twist. I would lay supine on our bed, naked and blindfolded so I would not know which friend I was helping. Her friend would lower herself onto my face and stay there as long as she liked. My wife would do whatever she wanted to me while I helped her friend. Her friend could stop at any time, take a break if she wants, and change positions as often as she likes. I however am not allowed to stop performing oral until I orgasm. If I orgasm too soon, her friend will be disappointed. My wife will be disappointed. My wife says I won't get to go down on her for a month if I disappoint her. I'm thrilled to be offered out like this, but what if I fail? Just what does she want to do to me that would make me lose control too fast? Gosh, which friend is it? If I do a good job will I be offered out again? Should I agree?