Here's a pretty good calculator to get a rough idea of what it may be worth. It'll vary from Craigslist to ebay, etc, but I've found it a pretty good starting point for getting a idea of the value.
That white MacBook started I think in 2006, and was discontinued around 2011. You'd need the specs - if he could email them, he can find them under the Apple menu -> About My Mac. If you can find out the processor and memory it has, that'll narrow down about what year it is.
I checked on the website above and the earliest model, from early in 2006, with the lowest specs, is valued about $210. You'd need to find out more about the specs on the one he is selling to get a better idea.
Also, the Wikipedia MacBook page has every model and year and what all the specs are.
Also, you might find out from him if the battery is still decent.
My girlfriend still has that Macbook from later in 2006 and it's still going strong.