I am not sure LL Beans are "sealed" to begin with? Just some fast airbrushing using a water based paint? I think there were various makers of the decoys, so it does indeed vary.
With that said, I don't think you could reseal them without repainting. Dark cork really doesn't wick water like tan, but if it is not sealed and no bottom board as many LL Bean Decoys, the chines crumble out and pretty soon they don't look so good.
If he doesn't own many of them, I myself would hunt them out, shelve them, and look fondly at them remembering past hunts.
If he owns many and would like to continue to hunt over them, he would need to sand them first, then seal up with whatever sealer he prefers (heavily), and repaint.