Ok, how do we know if he will owe money to HMRC at the end of the tax year? From my understanding you don't have to file taxes here, unless you think you'll owe? Damn I think I have some homework to do on understanding filing/doing taxes here.
In general, you don't owe at the end of the year because your taxes should be correct each month anyway... so the majority of people on PAYE won't owe anything (although some people recommend that you should file anyway just in case - I've never bothered myself).
If you think the tax on your payslips might be wrong, you can just call the tax office and they will sort it out right away, changing your tax code and giving you a refund or deducting any tax from you that you haven't paid.
This happened to me a few years ago - I realised in June of 2006 that I had been on the wrong tax code for 11 months and had paid almost £2,000 too much in tax... so I called the tax office and they changed my tax code immediately... and 4 weeks later I received a cheque from them for £1,500... then for the next 6 months I received tax credit in my payslip each month until the full amount was paid back to me.
Also when I was a student, I was working in the school holidays and was paying tax when I shouldn't have been (as I wasn't working in term-time), so at the end of each summer, I just sent off my P45 (from the job I'd left) to the tax office and they sent me a tax refund a few weeks later.