When you arrive at the check in desk, it doesn't hurt to have a letter (doesn't need to be from both parents / guardians, one is fine) stating that you give the 'UNMIN' - unaccompanied minor - permission to travel without an adult.
You will then be asked to fill in a Form of Indemnity which removes any responsibility from Ryanair's side of things, including a signature from parent / guardian.
Obviously if there is 2 UNMINs travelling, you should bring a letter from the friends parent as well, not sure if their parent / guardian will need to be there to sign the FOID as well? But I'm sure if the check in agent can see the girls are friends and travelling together you may be able sign for both UNMINs... to be clarified though.
Hope that helps!
