I know I'm jumping in on the conversation a bit late, but I have a few ideas.

Let me see if I get this straight (The Cliffs Notes Edition), and then I'll ask a few questions I don't think anyone asked yet:

Old friend of your GF gave her extacy (an illegal narcotic)

He lives in Canada - first question... is he a Canadian citizen or US?

Where did the alleged transaction take place? State of NY, USA, or Provice of Ontario, Canada?

If he's a US citizen living in Canada and the transaction occurred in your county, you need to contact your police. No joke. If you know where in Ontario he lives, give the info to the police and they will turn it over to your prosecutor. The prosecutors office can make arrangements with the DEA and Canadian officials to have him extradicted back to NY and face all kinds of drug posession/smuggling/distribution charges. They might even get him on fleeing the country.

If he's a Canadian citizen and this happened in the US, he better not show back up here again. Give the police his info the next time he comes back to the states, and they just might pay him a visit.

If he's Canadian and the transaction took place in Canada, do the same thing only you'd have to drive up to Canada and report it to HIS local police.

Here's the thing, Chris: you have knowledge that he was (perhaps still is) in posession of a certain amount of an illegal substance. That makes you a witness to a crime. Your girlfriend, as the victim, would not necessarily have a say whether or not to prosecute him. That's where you need to step up and tell the police what happened.

You said you wanted to mess him up somehow, right? Let the tight noose of justice do it for you