I don't know. I guess if they really wanted to- prosecutors could bring charges against him based on eyewitness testimoney and his emails- but it would be unusual to do so without physical evidence in such a case against a user.
NO they could noit. It's absolutely impossible to arrest someone on drug charges based on someones saying they witnessed someone buying drugs. I believe that's part of the 4th ademendment. That's why all this media speculation about when or if Rush is going to get indicted is so silly.
Even if they had tapes of him during a buy without being caught in the act of buying, selling or in possesion it means nothing.