That's so awful!!! I only got to talk to him a couple times, but he seemed like a really good dude. I hope his family can get through this and keep going.
Buy a set or two of wheel chocks. Even the cheap plastic stackable ones in the trailer section are good. Just keep them behind the seat, you need them anyway for changing a flat, or having ebrake / trans issues. Place chocks both forward and behind the wheels opposite the jack.
If you're going to break a wheel off the car for any reason, lay it flat and slide it under the cars rockers. It's bad enough just having a jack go off-axis with just a tire change. Especially do this if you're going to crawl under the car. The car "only" falling to 'Width of Rim' off the ground gives you, your friend or family member, and first responders a lot more to work with.
Do NOT EVER use concrete blocks with the lightening holes in them to support anything. All of their strength is in the web direction, they fail suddenly and without warning, they can't tolerate point loads, or off axis loads... Look you can break these blocks over a dudes stomach without hurting him, they won't even slow down a car.
Lift the car to the needed height, and place your jackstands with good footing, set the locks. Ease the car down fully on the stands. Wiggle stands to see if they feel secure. Stand back up, SHAKE the car. If it's not solid, you are NOT jacked. Try again, add jack pads below stands, cribbing...if it wobbles, just don't.
Once the car is firmly seated on the stands, and everything feels secure, bring the hydraulic jack back up to a little bit (couple inches) Below the jacking hard point. I don't expect the o-ring in the jack to catch a car in an accellerating freefall. We just want it obvious where it needs to lift, and ready to start pumping.
If you need to do something that takes a sketchy amount of torque, find a way to bring it out from under the car. A couple feet of impact-rated 1/2" drive extension coming out to rest across another jackstand sitting on cribbing next to the car can take away the leverage that was going to make the car shift on the jack stands
When someone comes across a person that's pinned under a car, don't make them have to guess or work to find where these stupid jacking points are hidden! Most people don't do well under stress, help stack the deck for you, and your rescuer.
[This message has been edited by Jerb (edited 09-08-2021).]