What a wonderful question. I wish people had asked me back in those days instead of just assuming my son's behavior was due to lack of parenting. So I'll add just a few:
1. Don't say, "Give him to me for two weeks and I'll fix him." Don't recommend the latest fad in parenting strategies. Believe us when we say we've tried all of those things. Because we wouldn't be living like that if we could "fix" our child with a star chart.
2. Offer physical help. One of our very worst years was when my autistic son was 4 and my other two were 2 and 1. If my autistic child refused to get in or out of the car, I couldn't physically "make him" without setting the babies down. Sometimes I did; sometimes I just stood there with one on each hip and cried. You don't want to pick up someone's child without asking, of course, but asking "What can I do?" during a public meltdown can be a godsend.
3. And then there's all the detritus -- diaper bags, sippy cups, pacifiers, toys, etc.,-- that we had to leave behind to get all three kids safely in the car. Picking up all of that stuff and bringing it to us would give you sainthood status in our eyes.