I've read the German and English Wikipedia on this topic as well as the translated Belarussian version and some sites I googled.
They all seem to argue that the 'Rus' part stems from the Region called Ruthenia.

The German version advocates that the 'Bela' part might derive from a word meaning 'western' (as the western part of the Rus) whereas in the English Wikipedia it is translated as 'white'.

Also according to the English version the name originated in the 13th century.

I have a colleague at work from Belarus and he says something completely different. According to him the country was named after a beautiful(~bela?) girl called 'Rus' around the year 800.

Does somebody know where his believe comes from and what the more likely/true explanation is?

My colleague's explanation sounds like a conspiracy theory to me. Only the historians who loved their country explain how it really was. And all the information I find on the web was either misinformation spread from the current government or propaganda left from the time when Belarus was part of the Soviet Union.