mikep,
make sure your insurance covers you for this type of work!
Before you even attempt to start cleaning it, take the electric fuse out at the consumer box, and cover the surrounding area in plastic sheeting, you should construct a mini scaffold tower beneath the chandelier with stablizers (depending on room), and tie strong plastic sheeting to each corner to form a catch for any falling crytals that accidently fall, and either work from the tower itself or use step ladders.
First, get yourself a few cans of compressed air and blast away any loose dust from the top of the chandelier and work your way down, then cover each light bulb and electric wiring in plastic food bags tieing with elastic bands and wrap in masking tape to secure.
Mix ammonia and distilled water in a garden sprayer bottle and do a small test spray on the chandelier to see if there is any reaction, leave for 10 minutes.
Once your confident, go ahead and spray the rest of chandelier and leave for 10 minutes, then spray rinse with pure distilled water (don`t use tap water, it will leave marks), then finish with another can of compressed air to help dry it out a bit, finish wiping with a microfibre cloth and remove the plastic food bags from the light bulbs and give them a wipe with a damp rag soaked in the solution. this will take a whole day to complete.
Tell the customer not to turn on the light for 2 days for drying out reasons and safety.
job done
let me know how you get on!