Seeing the other posts in this thread, I felt the need to comment...even if I'm probably not qualified to give this sort of advice.
I don't really see the problem here. If you feel like you are at a point of understanding and equality with your wife, then it's okay for her to be with Ahmed, but only as long as they both agree that it's not a serious relationship. Because of his religion, he won't have sex with her unless they're married and since she's already married, then this is impossible. Unless he renounces that part of his religion (or his religion entirely) then the relationship will probably not last very long since there's no possibility of sex or marriage. It's very important that everyone understands this.
It's also important that your wife not try to seduce him that way, or give in at the first sign of interest from his part. If he really does believe that sex before marriage is something he shouldn't do, then your wife doing that with him could cause some major conflict. He might change his mind about it later on, but it's definitely NOT something that should happen in the heat of the moment.
I don't feel like it's so bad...but tread carefully. It's real easy to go wrong in this situation.