Pilot displaying dangerous behavior.

[Rushie]

If you know anyone who has successfully gone through this I would love to hear from them.I am trying to do something good for someone who is descending into the grip of addiction.

I know of one case where someone successfully did this. Contact the FAA as directed by bbchien and they will take your statement. I believe they prefer to know your name however will not disclose it to the subject. They do their own investigation, they don't just yank the pilot's certificate without verifying whatever it is you tell them. In the case of which I'm aware, the pilot did suspect who turned them in but figured that out from circumstance, not because the FAA revealed the source. So you must consider you may be "made" anyway. But in this case, the pilot was de-certified and is now safely grounded, angry, but alive, and more importantly any innocent potential victims will live. They'll never know or appreciate that you saved their lives but you will, and you won't regret it.

 

[Jeffythequick]

Having dealt with this kind of situation before (not in the pilot world, but in other things*), to the OP, do this:
1. WRITE DOWN the facts of the case. If you do this from memory, you're screwed. i.e. "On 12/25/16 3:00am, the person came down the chimney with a bag of something. He was wearing a red fur suit with white trim. At 3:01, he dusted himself off, and proceeded to the table and he took cookies that were left there and ate them. He then took out of his bag some wrapped boxes and put them under the tree."
No opinion, just facts. Opinions show bias, and facts will be presented to the person in the investigation. Written statements made at the time of the incident outweigh recollections in court.
In your first statement, you said that the pilot was "on drugs," and unless you saw the pilot actually taking them, you cannot testify that s/he was on them. I'm not disbelieving you, but taking the opposite view, and what an investigator or lawyer will do to you. "The guy was on methamphetamine" when you've seen people you know on the stuff acting the same way is way different than "I saw the pilot, in the restroom take some pills (I have no idea how methamphetamine is distributed...) and 15 minutes later, he was throwing full coffee pots around the pilot's lounge, looking for a yellow and purple striped mouse."

2. Present a copy of these facts to his/her supervisor. Anyone else that was around will either agree, refute, or provide a different view of these facts.

3. If #2 doesn't work, escalate to upper management, and then to the authorities.

*For the details I was dealing with, it was custody of my 4 older kids. My ex had to drop off the kids at where I was staying, and she had the 5 of them, a friend of hers, and her 2 kids in an F-150 super-cab. I made a form for her to sign each time she dropped off the kids, stating who was in the truck, and that the interactions were non-confrontational. She signed it, I signed it, and a witness (sometimes her friend, sometimes one of my friends). After 6 weeks of this, I presented these signed forms to a lawyer. He told me that having her sign these was the best way to get custody of the kids, since she was driving a vehicle with 6 seat belts and 8 people in the truck, and her friend wasn't a good witness for her, since my ex and her consumed 2 of those seat belts, and the 6 kids had 4 of them to share, essentially putting their lives at risk.
Facts, not recollections.