While I don't know why, you are obviously not comfortable with doing his vehicle at your house.
The reasons are yours and don't need any explaining, but....
If you aren't comfortable with it,
don't do it. 
As a general rule, our gut instincts work pretty good for us.
I don't see your location in your profile, but could this be a time to consider a rinseless wash at his place?
One or two gallons of water is all you would need to take with you.
If the vehicle is really cruddy, one or both of you could take it to the DIY spray wash and knock the heavy stuff off there.
That might keep both of you happy.
Just an afterthought, could he be trying to keep the detail a secret from someone? Maybe spending the money for cleaning up the car hasn't met with approval at home.