Well, there's two ways to look at it. If you do as the client wants, and remove all water from the system (which isn't possible) and then add anti-freeze, when the anti-freeze evaporates there shouldn't be any liquid in the system to freeze and cause damage. However, as has been stated there are other ways the system can be damaged through non-maintenence.
Second way, if you leave a little water in the toilet bowl and tank, then add a good amount of anti-freeze to the water, the liquid will not evaporate as fast and the anti-freeze will stay in the system longer. Plus. you must leave enough water in the bowl to prevent a back flow of sewer gases. (which is why a toilet is built the way it is, to trap water and prevent the back flow.) sucking the water out of a toilet with a shop-vac is a waste of time, IMHO. Just pour the pink liquid until it's all pink.
I will state here that there is no guarantee/waranty - stated or implied on a winterization. But, you can and will be held accountable for any "freeze" damage found at the property after you have completed a winterization. Best bet, do the best you can, don't scrimp on the anti-freeze, and TAKE GOOD PICTURES.