Does leaving contact lenses in for an extended amount of time harm your eye?

Yes there is. If you leave contacts in your eyes for a long time they can melt and get stuck to your eyes. Then you would have to get surgery. The doctor probably told her when to switch her contacts. You shouldn't keep them on 24/7. Take them off at nights, and switch the contact lens after a month or so. If your mate doesn't like contacts tell her to get lasik surgery, much easier but much more expensive.