More complex user permissions/ roles

Can it be done, and is there a plugin of some sort?

I ask this because I have done some web design for a completely and entirely annoying individual that LOVES to pop in there and edit things.. and when they are faced with a bunch of code, instead of ASKING ME they just edit away.

I basically had one page on their site coded so that it looked nice and had a cool javascript effect for the frequently asked questions.. well, now all of that is undone because this person can’t bother to READ THE INSTRUCTIONS I GAVE HER/HIM

Anyways..I’d like to be able to give her permission to edit SOME pages, but not others. Can I do this? Any suggestions on the best ways to go about doing this?

Please help so i dont end up killing a client.