I don't think you can validate a German Rail pass in France. Kehl is the first stop in Germany after leaving Strasbourg, and the fare from Strasbourg should be around €3,50. Unfortunately, express trains going up and down the Rhein don't stop in Kehl, and I suspect few express trains from Strasbourg stop there either.
The next stop, Offenburg, should be a stop for almost all, if not all, express trains on the Rhein route or some of the express trains (although not TGVs) coming from Strasbourg, so it would be the best place to get off and get your pass validated. You'll likely have to change trains in Offenburg anyway. The fare from Strasbourg to Offenburg is around €7.