Adolescents are becoming more reclusive. This isn't surprising to me. Here's my advice: If your buddy's too much of a puss to talk to her himself, you could act as his wingman aka speak to her for him. If that proves unsuccessful (or you just don't want to do it), offer to him the suggestion of first communicating to her through text-based interaction, such as AIM or Myspace, which may make it more comfortable for him to approach her in person.
Assuming none of that works, just drop it. Don't worry about him and let him deal with the problem on his own, whether that be by having the balls to talk to her himself or simply going on as he is.
