If the phones aren't on her account, it's illegal. (In the US it's a violation of anti-terrorism laws, illegal access to a computer and a host of other federal violations.)
If it's in her name, "Let me have your phone for a minute" coming out of her mouth is the only thing she needs. It's her phone - they let her see it or she shuts it off at the carrier. (And don't tell me about kids not wanting to be spied on - I raised 4, and one of my daughters is now raising 2 teen-agers, and if they did anything we couldn't know about until they were 18, they got stopped. And they all turned out fine [except the 2 teen-agers, who haven't "turned out" yet].)