As I'm sure you know, it is a criminal offense for you to engage in illicit drug use whether or not you are on Probation. You are supposed to tell your probation officer of any contact you have with the police. If you tell your probation officer that you were using marijuana, however, not only can he violate you for that, but he can impose a drug testing program on you and take other steps to keep you from reoffending while you're still on probation. In my experience, something like this where no arrest occurred would not result in the termination of your probation, only in sanctions.
The officer who didn't arrest you still could. He probably won't charge you if he charged your friend. But he has until the statute of limitations runs out on that crime to do so. As he will turn the case information over to the prosecutor so that your friend can be prosecuted, it's possible that the prosecutor can find your name and your record and notify probation. How likely it is, on the other hand, that probation would be notified is something I cannot possibly predict. I suspect the odds are in your favor that they won't learn of it, but there are no guarantees.
It is best that any report of this incident comes from you and not from the prosecutor's office. You're probation officer would give you more credit if you come forward yourself. On the other hand, he would be still be unlikely to revoke your probation for this first violation, since it did not result in your arrest. He would likely just impose sanctions.
As a lawyer and an officer of the court, I cannot tell anyone to break the law. You'll have to do what you think is best. All I can do is put all of the general information out there so that you can make your own decision as to how to proceed.