Just ask the ET office to put the return ticket on standby. Once your friend decides what date he/she wants to travel, you have to pay a change fee and as previously poster's have said, potential fare difference. Have done it several times, even as recent as a few weeks ago.
ET is extremely flexible when it comes to moving dates, as long as inventory is available. I have many times moved a ticket and paid less in change fees than what it says in the fare conditions, as far as I can see. Often about $ 50-75, even on the cheapest fares.