Gonna be completely honest OP. You are really being a jackass to this person. I understand you may be displeased by the length not matching, colour even and possibly the nails. Though you have to consider most people can barely afford repairs. Also as for the nails it’s better to be safe. Get longer than shorter ones.

Have you considered just discussing the issue with them civilly. Suggest to possibly replace a few more parts of the fence or it fully as the new stable wood he put in.

Then show him how to properly replace fences. Also possibly if you have a saw for wood. Just let them know if he’s ever doing projects. He can ask or rent it from you?