In reply to Cragrat Rich:
There's tons of good advice on this thread. Here's a way of structuring it which may be helpful.
I'm assuming your friend is trying to onsight and hitting an onsight ceiling.
Have 6b (or whatever it is for him) as an onsight grade.
Try something 2 grades harder, e.g. 6c, as a redpoint (worked) grade.
Forget onsighting (for now). Just get on some redpoint projects on a top-rope (horrors!) and work them, resting on the rope, all over the place.
Can your mate do the moves? If not, is it:
Lack of technique?
Lack of finger strength?
Lack of power?
Lack of...
Whatever it is, training needs will emerge. The answer (as many have suggested) will probably be bouldering.
If your mate can do the moves but can't link sections and do it in one, on a top-rope, it looks like:
Lack of power/endurance?
Lack of endurance?
Again, clear training needs...
If he can do it on a top-rope but can't redpoint it:
Lack of bottle?
Lack of self-confidence?
Lack of tactics?
Again, clear training needs...
The trick is to use the redpoint grade to translate the unknown ("I'm failing - why??") into clear training needs.
You keep moving the redpoint grade - and the onsight grade - up.
A couple of last points:
Serpico was giving your mate really good advice - it's little by little.
Enty and Mark are spot on - you have to put the work in. Unless your mate is doing something climbing-related at least three times a week, he may get to 6c but it's unlikely he'll get much further. Don't want to be negative - just mean to be honest.
Hope this makes sense. Good luck!
Mick