Call a locksmith? A garage specialist?

Sounds to me like you're going to be cutting a hole in your garage door. Replacement panels are often available from the door manufacturer.

The make a locking release system for garages like yours. Basically it pulls on the emergency release cord of the opener.

Good luck!

Hopefully your car is on the outside?

Anyway, similar to what the previuos poster recommended.
You can cut a hole near top center of the garage door with a hole saw. This might allow you reach the release cord. You may have to get creative with a wire coat hanger and a broom handle or something to reach it though.

If this doesn't work, remove the entire panel.

Well, I hope he got in by now, but putting a hole in the door will cost money to repair.
If it was me;
I would strip a few panels of siding off the garage wall and then cut a hole in the sheeting behind the siding just big enough to crawl thru and open the door. Then the siding could be put back on and there would be no cost involved other than personal labor.... and the prospect of trying to get a panel to match an existing garage door (with fading and such) doesn't sound like any fun.

rharvster you are assuming he has siding. What if his garage is stucco?

A door technician would drill a hole about chest high in the door and using a special tool would hook the rope and release the door. Then he would install a locking mechanical release in the hole to prevent this happening again. Not cheap by any means but look at the other options. This should have been done when the opener was originally installed but no one wants the additional expense.

Hopefully, after 6 and a half years, they were able to get into their garage...

And the award, for "Digging up an old thread for no good reason" goes to...

I'm guessing that spam dug up the thread and then kirkhall commented on it. Spam seems to be a bigger problem now that Houzz has taken over. It gets removed quickly, but it seems to get posted more frequently.

lol. That makes sense.

LOL, hurray for old threads. Time to install a door to the garage. Either a door to the garage or a garage door with a keyhole on the outside are pretty much something you should have. Sounds like someone did a DIY without thinking of a failsafe.