A good chance at what? Being acquitted? He might have a better chance if he were willing to produce any evidence on his own behalf, but from what you've told us he's likely going to take the Fifth at trial and he's refusing to produce witnesses who will contradict the statement of the alleged victim.
Your friend can talk to his lawyer about his chances at trial and why the prosecutor might not have charged, or might be delaying a decision on charging, the (confessed) drug dealer.

