I agree with jfoofj: if the parents are paying for the phone, it's the parents' phone, and they get to see all texts they want to see. That's how it works in our house - and I do read, or at least skim, most texts on DD's phone.
Not b/c I don't trust her, but because I don't trust other kids. It's a great guardrail for her to be able to tell her friends "don't send me something you wouldn't want my mom to see." I don't get this notion of privacy that seems to assume our kids automatically will know how to handle any situation that comes their way. I've been glad to intercept some texts from bullying girls, and be able to discuss them with DD. Not sure I would have seen them if we had a don't ask, don't tell text policy. Also, kids text stupid stuff that they don't realize could get them into trouble. We've had some great teachable moments come up over the texting. Why would I leave that kind of education to peer groups and getting burned?
Bravo to OP's friend for wanting to be proactive with her DD. Of course, if you're going to read teen girls' texts, you must be ready to be bored to tears: Hey. Hey. Wat u doin? nuttin. O. whtru warin to skol? hey.