I have a pitch witch and in my experience will not remove moss,this particular piece of kit is designed toward cleaning the shingle type roofs in the U.S where pressure cleaning is detrimental to shingles.I have converted mine to clean slate.Cleaning roofs is hazardous,I use life lines,carabiners and the like.Remember Health & Safety requires you are taking all reasonable precautions to avoid falling when working over 2 metres,scaffolding or tying off too will not work if you are planning to walk the roof(not recommended) if you can e-mail me a pic I could assess the roof for you.As regards preventing the moss coming back discuss with your customer what his budget is.I have a few solutions for you to discuss with him/her.The moss has appeared due to the fact his roof has a drying out problem but without a few more details ie the roof tile type I cant offer more advice
Kevin