This strikes me as a powderkeg of a situation and if you are motivated (or preceived ) by anything other than genuine safety concerns, you could find yourself alienated within your company. The type of operator will be a large determining factor how to proceed (if at all).
Big operator with good structure? follow proceedure with FSO. Small operator? maybe quiet word with boss (be aware, bosses loyalties lie may with most productive staff or ones with loyal work history,)
I know that a lot of old flyers can end up sounding bitter at life but this does not take from their abilities and experience. And as for the falling down juice, well..... If he turns up sober for work, where is the safety concern there?
the drug accusation is more serious and unless you had concrete proof, thread carefully.
And as for mental stability? Nearly every rotorjockey I know has been referred to as "a bit mental" by someone at some point. (not medically trained to diagnose I should point out).
twice in my past I've seen good pilots being questioned (mental stability and lifestyle) by other less experienced pilots. Both times it backfired.
Big question you should ask yourself is "do you want to take a man's livelyhood away"? and "are you willing to risk yours in the process" ?
If you are 100% genuine, then get it in writting to operations manager or chief pilot as to your concerns (without naming pilot) as to companies policy with dealing with such situations.
NB