It is not possible to spy without the holder of the phone knowing. Plus even if you could get SMS messages (see below), she is probably using multiple ways to communicate with friends that do not show up on SMS texts. The only sure way is to have access to the phone itself with the pass-code to see all apps that can communicate.
There is message plus that you can set up on a computer, but the teen will get a confirmation code to approve the spying. Plus it is only SMS as I stated above.