Ideally, a sober companion should be someone with a good understanding of addiction and recovery. This person should be very astute and very observant.
A sober companion needs these attributes in order to effectively watch over the person that they are trying to keep away from using drugs. In essence you are guarding that person's life. Addicts are famous for being manipulative expert liars, and if you are not up to the task they will easily fool you. So that's why it is important for a sober companion to know the ropes.
As well, the sober companion should have naloxone available just in case of overdose. I mean let's face it, you are dealing with someone who has the potential to overdose and if you are not well equipped and prepared you could easily lose your client. There is no certification requirement, anyone can call themselves a sober companion but the reality is not everyone is suitable to be a companion.