This very thing happened to us at our local spot last September. In fact it happened two times out of three sessions, two weeks apart. First time was my friends 11m and the 2nd time it was his brand new 14m, both times was a strong SW wind. I was on my 7m fuel both times. (I'm a light weight).
Both times I stood helpless on the beach watching his kite drift very quickly down wind and both times he wanted me to fetch them but I told him I couldn't. I didn't know how and in my mind I didn't want to risk getting myself caught up in the lines trying.
Both times I suggested we pack up and drive around the lake (it's a big lake) to find it and that's what we did the first time but the second time he wasn't too happy so I drove around by myself trying to find it. I felt bad that I couldn't retrieve his kite but I have a feeling I made the right decision both times. There were no boats around, was basically entering fall season which was a shame cause there's normally tons of boats in the area.
He eventually found the 11m but as far as I know the 14m is still out there.
We don't kite together anymore. What can I say.