If he accepts a caution, then he will have a caution under the terrorism act. Whether that will be flagged on disclosure, I don't know.
I personally wouldn't, and would be more willing to take it to court. Ultimately, in court, you are given the opportunity to explain yourself, and explain your circumstances, all will go towards what the judge decides. No reasonable judge would knowingly ruin a guys career for the sake of a lark. I know one personally, and from what I understand, impact on the individuals future play a MASSIVE part, when doling out punishments.
There's also the possibility of not guilty for ANYTHING... depending on the number of charges brought against him, trespass etc.
I wouldn't worry, if its as innocent as it seems, which I'm sure it is..
Next time, tell him - when approached by security when doing something harmless which he thinks is legal/knows no better, Don't do a runner!