The normal performance model equations for slope effects are relatively straightforward and are available from a number of sources.
The main difficulty is convincing your local regulatory authority that the variations so calculated are appropriate without further flight test validation.
I am presuming that your colleague is looking at small GA aircraft in which case, if your State does not have a difficulty with operations from runways with significant slope, the exercise ought not to be difficult. You might, for instance, find that such an operation is limited to one-way takeoff and landing.
I note that you appear to be located in the UK in which case a quick email to Genghis The Engineer might give you appropriate guidance. He works in the relevant airworthiness area in that country.
[ 04 October 2001: Message edited by: john_tullamarine ]