I hope that doesn't turn out to be the reason for the conversation to have happened — it's a little unsatisfying. I guess another reason for H&C to need to talk to Zabini could be in order to use legilimancy on him. Presumably, in this case, to discover whether Zabini told anyone else besides his mother about H&C.

Edit: On reflection, this seems quite likely to me. H&C turns the conversation to betrayal, and if the plan is to use legilimancy on Zabini, then Zabini would need to be thinking about betrayals (and whether he has betrayed H&C) in order for the legilimancy to be useful.