Look at it this way: If the prosecution doesn't have enough evidence to convict a person, and that person has exercised his right to remain silent when speaking to the police, should the authorities be able to lay a charge with the expectation that the accused will talk to his lawyer, and expect that they'll be able to rely on those statements to the lawyer in court?
It completely undermines the right to remain silent and the right to counsel, if your statements to your lawyer are fair game for making out a case against you.
That said, such knowledge isn't without a consequence. If an accused confesses to his lawyer, that lawyer's duty to the court prevents him from being able to adduce evidence of things he knows not to be true. Thus, alibi evidence is off the table, evidence that somebody else is the guilty party is off the table...basically, any evidence that the accused is innocent cannot be adduced, because the defence lawyer would be knowingly misleading the court, and thus violating his duty.
Still, the lawyer still has a duty to challenge the prosecution's case against the accused. If the prosecution doesn't have the witnesses and/or physical evidence to prove guilt beyond a reasonable doubt, the court cannot convict, and it's the defence lawyer's duty to see that the court realizes that it cannot convict.
It it's not an admission, but physical evidence in the lawyer's possession, then other obligations kick in. A lawyer can't conceal evidence. (There are some grey areas if he intends to bring it up in court himself, but he can't just take the murder weapon and hide it away.) In Canada, at least, there's a practice trick called the 'brown bagging'. If a defendant brings you the murder weapon and tells you to deal with it (and refuses to take it away himself)...defence counsel can't just bring it to the police himself; it would be easy enough for the police to zero in on the suspect just by watching who he represents in court. Instead, defence counsel should put the object into a paper bag, retain another lawyer (thus invoking privilege with the other lawyer), and have the other lawyer bring the object to the police, telling them only that it might be relevant to some investigation.
As far as I know there's no reason John's friend's mom should need this information, I would suggest not making it available to her. John needs to seek legal counsel as soon as possible about what his options are. If he is found to have not been aware that he was transporting an illegal substance he should be okay but I wouldn't give out any information unless his attorney advises it's okay. In either case, legal counsel should be his first step because they can walk him through everything he'll need to know. He should make sure to ask as many questions as possible about his options. Best of luck!