Client disagrees with advice

We have advised a Ltd company client that an item he has claimed on his expenses is not allowable and if claimed will have to be declared on his P11D as an employee benefit. He disagrees as 'he knows others that have 'claimed it with no problems'.

Would you leave it on the basis that he is signing the accounts but keep a copy of your written advice to him in the file or ethically should you refuse to act as the accounts will be incorrect. We are not talking large sums (less than £600) but would be interested to hear others opinions. Thankyou.