I'd write back something like:
Hey XXX, it's great to hear from you. What have you been up to lately? Are you still [in job/college]? How's [the family/mutual friends]? I've been [whatever] It'd be great to meet up with you sometime. Are you free on [whatever day]?
Basically, a normal catch-up-with-old-friends e-mail. Don't mention the JW stuff at all. If they insist on pushing it, say you're happy to talk about your reasons for leaving if they're willing to listen.