Sounds like Tristan watches too much TV. There's not much you can do that may not offend your friend unfortunately. If her timeouts don't work then you need to be the bad cop and step in. When he exhibits inappropriate behaviour, you, as an adult - and a mother - should correct him if your friend isn't doing it. So, for example, given the car situation - I would have turned around and said "Excuse me Tristan? That is an inappropriate thing to say. You will not slit Emma's throat and I don't want to hear you saying things like that around Lucas. Do you understand?" Be firm so he gets it. Once he shows or expresses that he does, you can soften up and say something like "Thanks Tristan. You know, you're older than Lucas and he really looks up to you. I don't want him thinking killing is okay because it isn't. The things they do on TV aren't real, you know that right?" It's up to you to correct his behaviour if it's influencing or having an effect on your child. Failing that, Lucas will think Tristan's actions and behaviours are okay because you said nothing. Show Lucas that the things that Tristan does are not okay with you and that's how he'll learn. If the situation with Tristan doesn't improve, I would stay away from this family. You can maintain your friendship with Sandy when you're alone or when Tristan isn't with her. Good luck.