HMRC won't collect tax from employer

My client has a state pension and a couple of part time jobs.

A BR coding was issued to one of the employers in 2007/08 but was not operated. My client did not notice the error and by 5 April 2011 the tax underpaid is in the order of £2,500.

I asked HMRC to collect this from the employer. They refuse, stating that the employer says he didn't receive the coding and that my client ought to have known that she was not paying the right amount of tax.

I asked for a review, which reached the same conclusion.

Is it worth taking to a tribunal? Thanks.