What are some ways to make hanging out with an autistic preschooler less stressful?

I have a neighbor who has a 4 year old with autism. I think she’s lonely because she doesn’t socialize with the other moms and kids in the neighborhood because her son’s behavior is so unpredictable. I’ve been trying to socialize with her, but her son can be almost scary sometimes.

I have an almost 3 year old daughter and she is as chaotic as a kid that age can be. Her son gets whiny and kind of annoying early on. This doesn’t bother me. What DOES bother me is the meltdowns he has for almost no reason at all.

Last night he threw a fit because my daughter wanted to wash her hands after playing in the sandbox. She went into the bathroom and started to wash her hands. He screamed “NO!! YOU WASH YOUR HANDS AFTER YOU GO POTTY!! YOU DIDN’T GO POTTY!!” And he fell to the floor screaming and kicking and crying over this.

I worry that this will start scaring my daughter some day. Right now she throws tantrums herself now and then—so they don’t really faze her. She just ignores his tantrums and goes to play with toys in the corner. I don’t want her to get used to this sort of behavior though.