She may be looking for a covering letter in the style of a Query. Query letters are what the US agents prefer, and, in the US, are sent out before the sample chapters and synopsis in the hope of interesting an agent enough to request them. They are basically the same as a covering letter, but with a paragraph-long synopsis that ends in a hook line. So it doesn't have to include the ending, but does have to pique the agent's interest.
They can be difficult to write - one either has the knack or one hasn't. Nathan Bransford gives a good exampe or two on his blog (Anatomy of a query letter I):
http://nathanbransford.blogspot.com/
Also check out Forbe's synopsis in the S&O group, as that is the conciseness and tone you are aiming for.
- NaomiM