How to tell a boss his staff are rubbish?

This happened recently. I'm chatting with a guy exploring the possibilty of my doing some work for him in the future, and the question arises of how I would get on with the existing staff.

I, of course, tell him that there would be no problem, that I would be aware of the sensitivity of the situation, and as a result would ensure that all would be well in that regard.

Now, being familiar with the work of his organisation, it is obvious to me (and others) that his staff are a bunch of piss-taking wkers, that the work they do (when they are there) is poor quality and mis-directed. However, as the conversation continues it dawns on me that he doesn't realise any of this. As far as he is concerned, it's a great team and he wants to keep it that way. FFS!. How can I disabuse him of this misbegotten notion?

Basically, I'm already aware that there is nothing I can do about it (if I want the work, that is) but I just wanted to share with you the shock and horror I felt, and to provide you with an opportunity to mention similar experiences.