So do you not have any strop right now? It is possible that the edge might be able to be fixed with stropping, but that is much more difficult without a strop.

Anyway, stropping is always the way to start when trying to fix an edge and then working the way down the hones would be the next step. That is, try rescuing it with just the finishing hone. If that doesn't work then maybe the next one down will, but if that doesn't do it then you likely need to re-set the bevels.

Did you ask your cousin specifically what he did with the razor? That might help. Hopefully you had a nice discussion about boundaries after this screw up. Since he used your razor it seems fair for you to clean the bathroom with his toothbrush.