She didn't scream at him because she didn't know where her child was, she screamed at him because he agreed to lock the door and LIED about it and that resulted in her child being in a situation she felt was unsafe. He didn't have to know her parenting standards and adhere to them, he just needed to not lie about locking a door.
OP, I kind of think that it's your responsibility to watch your kids. I hold my friends newborn and still manage to watch my kids. If we're at someone's house then I watch them like hawks, if for no other reason than I don't want them playing with something they shouldn't and breaking it. If I go to the toilet I will ask whoever I'm with to watch my youngest and haven't had any problems, but if someone let my 1yo wander off then I just wouldn't ask that person to keep an eye on them anymore.
I can understand why you're angry, it's really freaky when we think about the things that could have happened to our children but at the end of the day, unless you are not there, it is your responsibility to keep them safe.