I don't believe it's a huge ordeal. Most likely he will have to get up to date on PDH's and pay a reinstatement fee and/or back yearly registration fees for the lapsed years. If Michigan is his state of original registration then they should be able to send NCEES a confirmation that he has passed the PE exam. I doubt that there would be a requirement to retake the exam.

License is issued by the State unless we are talking about that multi-state license deal NCEES does. So talk to them what they want. Yes, you probably have to proof he passed the test and met other requirement for license in the state. They likely didn't keep records from 30 or so years ago. the PE requirements also may have changed since when he originally obtained it and he likely will need to meet the newer stricter requirements.

A friend of mine had his "HVAC qualifier" license lapse. Actually his employer was in charge of renewal and let it lapse. So he had to re-take the test. So I wish your friend good luck that your State licensing board isn't full of a$$ hats.

Wishing you the best of luck. My experience, second hand, is that the state does not want to do something out of the ordinary like write a letter for NCEES and just tells the individual its your responsibility. My advice is to talk with someone at the state, going in person may help, and see if you can get a copy of the original license and the original license application. Tell them you will pay archival fees or what not. You can use the license and app to convince NCEES if the state does not write a letter.

I let mine lapse one cycle. Wasn't a terrible experience getting back to active status, but I was merely late at renewing. I was only in non-active status for maybe 3 months total. My state required that I fulfill the CEU requirements, provide proof that I had continuous insurance coverage, and a list of the work I had performed while in non-active status. Reinstatement was immediate once they received everything.

Generally, you'd just have to work through the rules and see.
I think in a lot of states, it takes a new application if the license is lapsed TOO long. Short times (a month or two or three), no problem, just pay extra money and send in the paperwork.
I would not think it took new testing, unless for some reason the testing originally done is no longer adequate. (For example, my TX license didn't require ANY testing when I got it, and I've got one other license grandfathered in without testing.)
The continuing education may be required to be caught up.

Obviously lapsing is the fault of the license holder. But I also lay some blame on the licensing agency. At least in my State. Every 2 years when I renew my license, I try to do it some months before the due date mid-year. but they only let you do it a very short time before the due date. In my state no documents are needed to renew, just a credit card for the fee. So it really should be possible to pre-pay. They also don't have an email reminder function.
I hold 2 licenses in this State, PE with 2-year renewal, and an HVAC qualifier with 3 years. And every once a while I get into panic and look up if I missed the renewal time (so far I haven't

For the HVAC qualifier I know someone who had to re-pass the test because of lapsing. So i never want to find out what my State makes me do to renew a lapsed license.

I let mine lapse in MD once and did not realize it. Sealed 6 drawings while it was lapsed. Was a total CF. They basically wanted me to get a third party to re-seal the drawings.
I can't remember how I got around that but it was not pretty and it cost me $1500