Cocaine Use At Work

The initial red flag I see is that he is using at work.  Even if you put aside the legalities of cocaine, when someone starts using at work, it is a bad sign.  It is an indication of poor judgment at best, addiction at worst.

Addiction is when the use of a substance or behavior negatively effects physical health, occupational performance, social relationships etc.  It sounds as though you have noticed signs- "erratic behavior and poor work performance".  You attributed it to his separation.  While that may be true, you may not know whether or not his cocaine use attributed to his separation.

Not everyone who uses is a drug addict.  That is true.  But when someone 's functioning is being effected, addict or not action needs to be taken.  This person's behavior affects you personally.  You have a vested interest in his behavior.  By not doing something you are putting yourself at risk and enabling continued bad behavior.  Think of the consequence should a client or staff member see him doing coke.  Would everyone keep the secret or would someone call the police?  I am sure in your company there are drug free environment regulations, work safety issues.  As an architect, does he ever go out to a building site?  If there is an accident and he is drug tested, what would the consequences be for all of you?

At this point it does not sound like it maters if he considers his use as recreational.  He is involved with criminal activity.  i know that is sounds like an extreme statement, but he is purchasing, in possession of and using an illegal substance.  Now he is bringing that substance into the work place.

Knowing that you will not get a straight answer from him leads you to confront him with the facts that you know.  What specifically do you consider his erratic behavior.  What specifically has been his poor work performance- changes  from past functioning.  He will likely blame it on his separation. The final part of the confrontation is the poor judgment bringing in and using drugs in the office.  It is a big deal.  It is not acceptable and it puts everyone at risk.  He will likely try to rationalize and minimize.  You may need to put in a consequence of some sort; telling HR about it, having him drug tested etc.  It would all depend on your position within the company.  This may escalate over time, so you need to be clear on what is important to you.