IMO, you shouldn't go on and on about whether her idea will work. That's NOT what she asked you to do, and that shows you don't know how to stay on task. I would, however, write one or two sentences about how some of your research indicates that her position may not be tenable under current law in the jurisdiction, but more research should done to explore more.
This flags the issue for the partner, and if she would like you do more research, she'll ask you. This opinion is based on the advice I got from a couple of friends I consider mentors (and work at large firms), so take it FWIW.
Oh, one other suggestion - if you're at a large firm, you may have law librarians available to help you. You may want to run down and speak with one, explain your assignment, the searches you've run (which you've hopefully kept track of!) and the fact that you haven't found anything at all on point. They can look at your search terms and database choices, and help you figure out if you've run a complete search. (Or if you don't have law librarians, you can always call the help lines for West or Lexis, whichever service you're using. Their research attorneys are very helpful for stuff like this)