Legal advice is not needed.
He should appear in court in person if possible, plead guilty (because he is) and offer his mitigation. He can explain to the Magistrates why he broke the rules (albeit by such a small amount), tell the court that he is remorseful and tell them he has learnt his lesson and it will not happen again. He does not need a solicitor to explain all that to the court.
If he cannot or will not attend in person he should write a letter to the court with his mitigation. If he fails to respond to the summons the matter will be heard in his absence, and a Not Guilty plea entered on his behalf. He will almost certainly be convicted and he will lose his discount for a guilty plea.
Although the maximum penalty is a fine of £2,500, Magistrates’ sentencing guidelines suggest a fine of half a week’s net income (reduced by a third for a guilty plea). In view of the very minor nature of the transgression the Bench may see fit to reduce the fine a little if this is his first offence. He will also have to pay a contribution towards prosecution costs of about £80 and a Victim Surcharge of £15.
As an aside, the court would not consider this matter ridiculous. One piece of advice your O/H would be well advised to heed would be that under no circumstances should he give the court the impression that he thinks it is, even if he does.