First, it’s YOUR business and the first step is for YOU to determine what your business continuity and disaster recovery needs and objectives are. Have you defined and documented those? If not, do so. A BC/DR is NOT just about the technology and the data.
Once you’ve done that you can present them to this person and tell him you need him to provide documentation regarding the technical aspects of your BC/DR plan that supports your BC/DR objectives.
If he’s unwilling or incapable of doing so, there’s no need to damage or risk the friendship over it. Explain to him why you need this and why you’ll need to get another party involved to handle it. If he’s a friend and a professional he’ll understand and be supportive of your decision.