If you have committed an offence, and the police do not believe that it is worthwhile taking you to court, you are cautioned. It usually occurs for minor offences, or "Misdemeanors" as they are called in the Sates. A fairly good example is possession of small amounts of Cannabis, or minor public order offences. I think it basically means you get a verbal and written telling off from a fairly senior copper. As an example, a mate of mine "Borrowed" a cherry picker in a bout of drunken buffoonery, and got away with a caution.
Not sure if that is a very exact definition, but I think it is close enough.
If it's found to be a class A, and a relatively small amount, he may well receive a caution from the bizzies. However, to get the caution he'll have to cough to posession. If he opts not to accept the caution, and the bizzies take him to court then the mags or the CC will deal with it. Either way it's a lose/lose thing for him.
He could always get his brief to try the "PTSD" option...