If no good can come of or be retained after your friend's conversion, then go ahead and try emotional blackmail.

Please don't. Becoming a Christian is not the end of the world. People are remarkably good at compartmentalizing, and a large part of the reason he's likely thinking of converting is probably that there's a large, supportive community waiting for him if he does. Demonstrate that he has a community if he doesn't convert, don't threaten to take away his current one. But emotional blackmail is not the way to go.