1) the police because they are the only ones with the authority, capability, and know-how to take care of the PUSHER(S). There is no doubt in my mind that a drug dealer is a rodef. I have life experience to stand on as well as some professional experience. But those who are uncertain are more than welcome to ask their Rav. While I don't care in the least for users, PUSHERS are a different sort of animal. Drug pushers are the ones who entice the young and seek to addict them. PUSHERS infect a community. And PUSHERS are often entangled in a messy and dangerous web. I wouldn't approach a pusher directly. I would absolutely seek professional advice from people who understand the drug trade and have some experience regarding pushers and dealers.
I can't remember ever commenting on the Japan bochurim case, but let's not forget that behind those boys was a grand PUSHER. Millions of tzedakah dollars and a lot of energy has been expended by kehillot all over the world regarding that case. A shame that the PUSHER was allowed to operate in at least somewhat of an open manner.
As for telling
One thing that I know should NOT be done is to approach the boys directly to befriend them in hopes of "changing their [lives]." Save you compassion and chessed for issues you understand and have the capability of dealing with. As my parents told me before going into high school, [and I'm dating myself, so younger readers, don't feel bad if you miss the reference], "you are not to go near, talk to, or be friends with anyone who wears a beeper." These kids had drugs and a scale out of the table. That is the signal to stay away! You aren't speculating about the use of the beeper. It is right in front of your face. I think PUSHERS are best dealt with by the police, but a person is welcome to talk to the baalei simcha hosting this mess, the wedding hall director, the (kashrut) mashgiach since the boys brought in outside drink, the boys parents, teachers, and menahelim. But befriending the boys; I give that the royal thumbs down.