Hi, I hope someone can help me out with this situation.

My daughter (15) and I have tickets (GOT-ARN-EWR) to return to the U.S. next month. Unfortunately, I can’t return that day and I can’t change her flight home so she will have to fly alone. I've tried to add Unaccompanied Minor service to her ticket but was told by SAS that it had to be added before we left the U.S on the RT ticket. (I had no idea I’d have to delay my return or I gladly would have done so.)

My question:

My plan is to take the first leg of the flight GOT-ARN with her then take the bus to the international terminal to get as close to the ARN-EWR departure gate as possible. Ideally, I would like to wait with her at the departure gate until it boards.

Is it even possible to do this, but not get on the flight, after I've already checked in and got my boarding pass? Can I go through passport control with her or will that be a problem?

Also, when should I notify SAS that I can't get on the flight? I don't want them to cancel my entire ticket.

Thank you for your help.