It is important that you let your homeowner know that in order to do your job, they have to let you, the trained professional, do all of the cleaning. When I have a homeowner that wants to do bits and pieces of their cleaning, I simply educate them as to why that is not a good idea.
In order to be able to guarantee your work for order, you must know how each and every item has been handled to ensure proper cleaning and deodorizing. You cannot guarantee anything that you do not handle yourself. And if those items that you did not handle are put back into the house and mixed with the items you did clean, they could re-contaminate the entire house if they weren't processed correctly.
This can be a little difficult to explain to a homeowner, as they may be thinking that they can clean their own things. Contrary to what many homeowners believe, deodorizing does take specialized training and, yes, some science to clean items that have been affected by a loss of some type to bring them back to pre-loss condition.
So one of the ways that you can help change their mind about being involved with the cleaning process is to educate them about exactly what that process entails. Talk with them about the specialty cleaning processes that you will need to use and your equipment. Once they understand exactly what is involved to remove all contaminates and odor, they tend to change their mind about cleaning anything.
I also want to mention that if your homeowner is adamant about cleaning some of their items themselves, you should have them sign a release form that states you are not liable for any odor that may be present once all items have been reset in the home. Since you did not handle the entire cleaning process for all items, you cannot give an odor-free guarantee.