First, don't use private student loans.
Private student loans are vile. They have all the bad characteristics of most private loans -- but they also have special status that they cannot be discharged in bankruptcy. The high interest rates is what made that debt unmanageable, and the responsible thing to do in that situation is discharge it in bankruptcy. However between sticking her head in a noose by taking them in the first place, and sticking her parents' necks in the noose with her, she became the author of her financial ruin. Which she will overcome, she just won't enjoy it none too much, which she should not.
The parents, by cosigning, also deserve some of the pain and should be helping her pay the notes down.
She should also be paying the highest interest loans first, but I'm sure the lender has rigged it to make that impossible.
Pay interest and principal on separate cheques
The other thing she definitely should do, is write two separate checks for interest and principal. That will require doing the math to figure out how mhch interest she is accruing each month, and based on the variable rate it may require a monthly phone call to the lender. Alternately, she can make the interest payment definitely large enough to cover the interest.
Computing and paying the interest separately assures you are paying 100% of the interest every month. Which is a really big problem she has been having. The design of those loans allows them to go into neg-am, i.e. The lender will accept a minimum payment less than the interest charged. She must make sure to pay 100% of the interest every month, because they won't.
The second check, for the principal, is designed to make the paydown real. It educates her that it is this payment, not the other, which is reducing her debt - and only this payment has that effect.
In fact, that is correct accounting practice - in GAAP, every loan payment must be fractioned into interest and principal, with each one treated quite differently.