We met at the library with him and his mom ( who is a good friend of mine), then went to lunch together , then a playground.
He chose to ride with us in my minivan for one ride in between locations. Ds1 rides in a booster, his friend rides in nothing. There is no booster law in my state. Stupidly, kids can ride in a seat with no booster or car seat at age 4. (ETA: the boys involved here are 8, I mention age 4 to give a sense of the car seat " culture - or lack there of- in my state)
Anyway, what he does in his moms car is their business. I offered him the option to sit in our extra lbb. He said no. He is a larger child than ds2, but I was horrified when I turned and saw how unsafe he was due to how the belt hit him.
I was already driving when I saw and I kicked myself for not checking him before we left.
I nicely told him "the seatbelt might fit you fine in your mom's car, but it does not in mine. I want to make sure you are safe, so if you want to drive with us to the park after lunch, you will need to use a booster".
I do not believe that he is safe in his moms car with no booster, but that is her call when he is on her watch.
I am not going to knowingly drive a child in an unsafe manner in my car.
I would not drive another child in my car. Things happen, like forgetting to check a belt. That could have been a problem. My children ride with me. Other parents are in charge of their children. That's how I handle it.
Now that ds1 is older there are some friends I carpool with as having 2 kids in sports at once gets complicated.
I have not had the need before this year, but we are spread thin at times, so we "get by with a little help from our friends!" :)