Is it overkill to seek UCMJ action against someone for a first-time offense?

UCMJ is a command decision and usually based off leadership input. If his leaders felt UCMJ is warranted then that's their decision. Nobody here can second guess another leader based on a tiny snippet of info

I'm not quite understanding your narrative. I'm guessing an junior aircraft mechanic made an error installing a part. The error involved a bolt somewhere along the line. The part was damaged and probably had to be R&R'd causing the aircraft to be maintenance downed. This probably caused supervisors all up and down the chain of command to take some grief. As we all know stuff rolls downhill and your friend is at the bottom of that hill. He's covered in stuff and will shoulder the blame. Never mind that a Quality Assurance inspector should have caught the problem before the part was damaged.

Should your friend be punished under the UCMJ. That's really up to the commander. If I was the Maintenance Squadron Commander, I would make sure a complete investigation was accomplished. Based on the facts and findings I might choose to discipline your friend, his supervisors, and the Quality Assurance person. If a human error was made with no malice or intent to destroy Government property, then I would be inclined to ask the First Shirt for ideas on how to impress on the technician that following tech orders is critical to safety. Something short of non-judicial punishment or a Summary Court Martial.

I really don't even know what really happened - but yes they can seek UCMJ. If something is serious enough, it doesn't matter how spotless the record is prior to that offense.