You could rehome the one you buy when you're done as long as you find a farm with other ducks to take it, but then the one you're sitting will be very very lonely. You should either talk your friend into keeping them both, or rehoming the one you have. The one you have is a 15 year commitment. What is your friend going to do when she goes back to school again?
Tractor Supply has a minimum and won't let you purchase less than six, or shouldn't at least. I would find out where your friend got this one from and go there to get another. If it was Tractor Supply don't count on them selling you just one more because they shouldn't have sold a single one in the first place.