It's not difficult to check previous years' incomes - your friend can look at his correspondence from the tax office for those years to show what his gross income was, and also the net income would be showing on his bank statements (it's easy to get copies of those, as I am sure you know).
You need to google the HMRC site re time limits, but they can go back beyond 7 years according to this.
http://forums.moneysavingexpert.com/showthread.php?t=4565431
HMRC appear to be quite within their rights.