Um... that's wire fraud. Western Union has pretty strict standards about this, and he might have crossed the threshold. (I can't remember if it's $2000 or $3000 that makes them start to take notice.) Are you sure he *only* got $2400? Is he?
You should call Western Union pretending to be the friend of someone you think is being defrauded, see what they say about that.
I *think* he's relatively safe, for now, but there's no way he's gonna stop. Besides, how do you know he's done it to just this person? It's unlikely, and it's even less likely that he'll stop, it's too easy a scam. Get as far away from him as you can, as fast as possible, not only because he will scam you eventually but because when the feds DO come looking for him, you don't want to be caught either in their net or, worse, their crossfire.