Liability would be tiny I'd expect given where interest rates are. Remember, tax is only applied on the interest he/she received - not the capital amounts saved, which he would already have paid UK income tax on.
Tell him to check his account statements to see if interest was paid net of DIRT. I'm in the US and interest on our Irish deposit accounts are paid net of DIRT. We just keep transfering cash back into our old deposit accounts & never informed the Banks we were leaving. As such the Banks haven't change any of our account details it would seem - maybe your friend specifically opened a non-resident account.