Hello Vesseloflove,
I first want to apologize anyone made you feel attacked even if that was unintentional. But I do think it's cool that made these feeling known. This is something a lot of people struggle with. I don't think people always realize how things can be taken over messages.

If you do not have proof it is hard to go to the authorities. Even if you have proof of illegal activities it can be difficult to do that to someone you consider or even considered a friend. You could potentially let the police know about the interaction.

There could be other explanations for his behavior in this situation, he could be embarrassed about who he was meeting or afraid that you would think that was what he was doing there (trafficking). It is even possible he was doing something else illegal like buying drugs and didn't want you to know. I am not trying to speculate or say that he was not trafficking, just trying to offer possible other explanations.

I would try to talk to your group's mentors or supervisors about it. They may have better advice then any of us can offer since they may have experienced similar situations.

I do think it is appropriate to talk to your friends about this. They are also your friends and this could potentially negatively affect them. I would just be cautious of what friends you say what too. I would be concerned that if he is participating in this type of behavior, he may have friends who are also participating in this type of behavior. I would also be careful how you phrase it. It may be negatively received if it sounds like your accusing him. I would more try to be like "I ran into John the other day while I was with my trafficking group it was really weird...." and basically tell them what you witnessed and how he acted. Then just let them draw their own conclusions.

Most of all make sure you stay safe and sane. Good luck and keep us updated if you can.