Quatloos!

so one of my colleagues(UK-based) and long term friend received part of his income as dividends.

He's trying to find out how to calculate the income tax liability on this income. Having looked at various helpful websites he said he was still rather confused. SO, I thought I could get some help here!..

Can anyone help me on this?

This page on the Revenue & Customs website seems to explain matters:
http://www.hmrc.gov.uk/taxon/uk.htm

If the company is registered in the UK, it is likely that the tax will be deducted at source. If you need to fill in a tax return, there is a space to declare the income.
http://www.hmrc.gov.uk/forms/sa100.pdf
(See box 3 on page 5).