My employer knows my work colleage is on drugs what can i do?

I work for an company, that has a strict no drugs / workplace policy, however my direct boss knowingly lets one of the members of staff come to work stoned or off his face on speed ,he even goes home at lunch time for more drugs , he has been given a written warning once ,for going home to smoke pot before starting another shift. Now the boss has put him in her position while she has her RDO`s he is mostly a good worker except for the drugs which make him moody and hard to make sence of . On monday I had a very small argument with this guy over work (which he messed up) , now im concerned about loosing my job, when the boss comes back,what should i do? Do i have any rights ?