I agree with Everett, it is very important to find out the plans for future restoration. If they want just a cleaning for now, then do like Everett said and make sure you clarify your not responsible for failing stain/paint issues. For just a cleaning, I would down stream (or with a dedicated pump) a very light house wash mix, and rinse lightly using a low flow tip (black tip) or open ball valve, standing back away from the surface. This will help minimize any removal of flaking or failed stain/paint. It looks to me to be a fairly easy cleaning, looks like lap and gap siding, but hard to tell exactly from the pic. Always do a test spot, like on the back of the home before proceeding with cleaning. Take before pics of any preexisting damaged paint with something in the pic that shows clearly it's a before pic.
Anytime I have anything that involves wood I always find out if they have any plans for restoration, if they do I refer them to Shane or Adrian here in the Houston area.