I don't expect children to be as self possessed as an adult, but I do expect them to show a certain amount of responsibility for their behavior. Learning responsibility for your behavior starts young. If you take your sisters toy, you get time out. It graduates from there. If your 7 year old understands that, under no circumstances is he to go to the park without adult supervision, then I would expect him to follow that rule. Yes, children sometimes make bad choices, that is part of learning and growing. If we continually insulate them from making choices or from the consequences of those choices then we are not letting them grow and learn, and, as adults, they end up in the situation I described. How does a child learn to make good choices if they are never allowed to make any choices, or suffer the consequences of the choices they do make? Again, at 7, my kids knew that if there was certain thing they were absolutely forbidden from doing, if they did it there would be consequences. Saying to an adult "I am not allowed to do that, I need to cal my mother/father for permission," is not disobeying. My children also know that other peoples family rules differ from ours, some are stricter, some are more lenient, but just because X is allowed to do Y, doesn't mean you are allowed to.
I have been down this road, I have a follower. She was always letting other children influence her to do things she knew she shouldn't do. It did not negate her responsibility. She chose to break the rules. She could have stood up and said no, but chose to go along, for whatever reason, and those choices had consequences.
If I were the OP, I would not allow my son to play at the neighbor's house. Their rules on big things differ too much from my own, and my child lacks the confidence or the ability to stand up for himself in a situation where the possibility of rule breaking exists. He would still have consequences for breaking the rule. The next time the situation arises, and it will, he may think twice about not speaking up for himself. Just because the child's parents have more lenient rules, does not make him a "bad seed" or a "bully" and it is unfair to label him as such. I have friends who have very different beliefs about parenting than I do, some more strict, some more lenient. I have a friend that doesn't police the TV, and my child was coming home quoting movies I know she should never have seen. Now, I don't let her go there unchaperoned. I don't cut off the friendship, I just put certain limits on it.
I just don't think it is helpful to insulate children form all potentially difficult situations. I do agree that you allow independence in increments. I am not talking about leaving a child in the woods to fend for themselves, but being too overprotective either causes a fearful, dependent child, or a real hellion.