Eddie would be best qualified to answer this, but AFAIK, most insurance companies would be hesitant to underwrite a second driver who has no direct relation to the primary insured. Allstate was hesitant to underwrite us together prior to our marriage, but did so on a individual basis (they reviewed our history of coverage, both with the same carrier, and determined we wouldn't be an increased liability seeing as how we've maintained coverage with allstate for the year prior.)

This is just my example, however, I can see an instance where the carrier would deny coverage based on the fact that you're not family/married. If you look at it from their side, what would happen if you broke up? What if she didn't pay and that causes a lapse in coverage? Things to think about. It's like having a co-signer for a loan. They have to prove "credit worthiness". In this case, credit worthiness would be a direct relationship.

Of course, this is all speculation. Call USAA and find out and let us know. This might be useful to someone else on the board.