As you will know, the HMRC rates are currently 45p per mile for the first 10k miles then 25p per mile after that. That's the easy bit, that calculates the std mileage rate.
The other bit is going to depend on what sort and how modern a van he runs. So if he runs a new van and gets it serviced down at the franchised dealer and changes it every 3 years (and I know trademen who like to do that and I'm not knocking it) it is going to cost him (I reckon) more than the equivalent rate that the grey suits in HMRC worked out - so I'd be using estimates to quote at the CSA.
However if he runs a perfectly serviceable but 2nd hand van he bought at auction, gets a local mate to service it regularly and keeps it till it falls flat then changes it, then he's far better working on the 45p / 25p rate.