yeah, ok, you're slightly misunderstanding it.
In some sense, yes, it will use the styles from the "cache" - but you can't edit the cache locally. When he presses F5 the browser "refreshes the cache" so it fetches the file from your server.
So what options do you have;
First a couple ground rules;
a) firstly - if you have compressed and combined turned on then the designer would need to edit the root css file, but then run the gzipall.bat to create the all.css and all.css.gz files. So while he's designing it's probably best for you to turn combined & compressed _off_ until he's done, then you run GzipAll to squish them all back together.
b) the rules of "what" files he edits remain in force - ideally you create a single "custom" css for your app, and this is the only file he needs to edit. Editing the shipping css would be sub-optimal. (The only exception to this rule is if you use an alternative jQuery style as the style file for the app - in which case that file can be created, and then hand-edited if desired.)
next - some approaches;
a) The easiest approach is to give him a "local copy" of the site. ie something he can install on his own machine, and edit the css there - then send it to you when it's done. This is the fastest for him. Obviously this is easier for TPS than for SQL.
b) give him ftp access to the _css_ folder only. (Or if you like, the whole web folder). If you are gonna do this though, and you are making an SSL site, please make sure your certificates are _not_ in a place he can get to.
c) set your "styles" options so that the custom css file comes from a different location - perhaps from his hard drive. I'll need to experiment with this a bit to see what's possible. I'm not sure if the browsers will allow it (if they do it'll be way cool!)
I'll experiment a bit with option 'c' and keep you posted on that.
cheers
Bruce