Sigrid
Check out this link:
http://www.economist.com/research/styleGuide/index.cfm?page=805701

The Economist is a British publication - may or may not be helpful.

I couldn't find anything in Chicago Manual of Style that allowed for single quotation marks for dialogue.

I wonder if you could explain to your client that if there are going to be quotes within the quotes, for ease of style and reading you use double quotes for the dialogue and single quotes for quotes within.

Of course, you said she's insisting on single quotes throughout....and I gather, the client's the boss, right? [Smile]

Hmm...now I'll be looking for usage of single quotes only!