I think it only makes sense that the owner of the account should be able to view the text and picture messages online for all the phones on their account. They pay for the account and they should be able to moniter how the other people on the account are using their phones. If this is a problem for the other people, then they need to open their own accounts.
If you have issues or concerns ask the user for the phone to see what they are sending. You are the owner of that phone. and it's line.
Actually, they may be responsible for the account and all payments, but this does not mean they "own" the phone or that they even actually make the payments.
If the person "using" the phone paid for the phone, then they own the phone. Additionally, although the account owner is "responsible" for the account, quite often people will group together to start a family plan and each person on the plan makes payments for their "share" of the account. The account owner is only stuck with the payments if the others with lines on the account do not make their share of the payments.
Sorry, I made a bad assumption that if she wants to read texts and see pictures that she is refering to her kids phones. If she's not, then......... other issues.
Edit: her minor children, that's she's paying for.
I figured that was what YOU meant.
However, most parents, at least on these forums, mention their children when trying to find out ways to monitor the activity on a phone. Since the OP didn't do this, I would think that it is more likely a group of not necessarily related people banding together to form a Family Plan and receive the discounts associated with it.