Ditto, ditto, ditto!!!! Absolutely NOT too strict. In fact, I think you handled this beautifully. Not only does teen have to deal with facing what happened on her own when she returns to the couple's house, but she also knows that there really are boundaries with that freedom teens cherish so much.
I think you are allowing her to learn a very valuable lesson and more than one great thing will come from it. She will more than likely make the couple happy because she cares enough to go back, which they probably aren't expecting. She will also learn that it really does feel good to do the right thing.
I think your solution is an all-around winner. While she may come home to you in a huff over the humiliation of having to return to the scene of the "accident", I doubt she will be anything but humble and apologetic while meeting the couple.