What can someone with 30 years experience in software do after retiring

As a retired software developer with 50 years experience I can say that he's unlikely to get hired. The software business is very age-biased. And, since you mentioned that he was the "director of a center", that implies that he "retired into management" rather than actually continuing to develop software. Plus, as a manager there is no time to keep up with the leading edge. In software, especially now, as the Red Queen said: "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

However, retirement is amazing. I spend most of my days on my own projects, which usually involves taking several courses on youtube. For instance, I'm working on a problem that requires a camera so I'm watching "The Ancient Secrets of Computer Vision" (https://www.youtube.com/watch?v=z5WSV6CXsxs&list=PLjMXczUzEY...). Nobody would pay me to learn how to compute the robots grasping pose of a 3D printed object from its GCODE description.

Retirement is great fun and the perfect opportunity to spend 18 hours a day, 7 days a week (modulo random naps :-) ) hacking on whatever software that strikes your fancy. Enjoy it while it lasts.

This is kind of an anti-answer, sorry about that. I confess, after 40 years of software development, I was done with the technical world. In the three years I've been retired, I've sailed a lot, done woodworking, made music (guitar, piano, voice, played a lot of chess, exercised, and hung out with my wife and the kid I mentor. If I had more time, I'd improve my Spanish. I have travel plans, too. Does he really need to make more money? Has he no other interests?

The natural progression from retiring from a job in the government tends to be taking up a new job with the government or a government contractor.

This is one place he could look:

https://www.usajobs.gov/

A better path might be to network with former colleagues who work in areas he is interested in. Contract pay in government work tends to be higher.

Outside of government I would say large corporations. Again networking is going to tend to work better here than applying directly if possible.

I mean... is he retiring or just "changing jobs?" If he's retiring, I'd say what he can do includes:

Go fishing

Take up photography

Take up a musical instrument

Learn a foreign language

Travel

Go camping

Volunteer for a cause that's near and dear to his heart

Read

Attend live music performances

Go fishing

Go to museums

Restore an old car (or tractor, or bicycle, or radio, or $WHATEVER)

Build a Tesla coil

Go fishing

etc...

Software testing; his experience would enable him to know which specific tests to perform in any/all program areas.

figure out what slice of reality your dad is "the" expert on and focus on that audience

Videogames? Indie game dev, etc.