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You are under absolutely no legal obligation to pay the roommate half his deposit back. He is the one who is breaching the lease by first smoking and then leaving prior to the expiration of the lease. So legally, if he moves out without your permission, you could sue him each month for half the rent. With that said, if you are agreeing to release him from the lease, then it is the landlord who decides whether or not to refund half the deposit, not you. If you are both on the lease, the landlord has to divide up any refund equally between you both and if you have already given roommate his half, he could get a double recovery.
So I would simply say "No" if he wants half the deposit back, he can take it up with the landlord to see if he will refund it. You are already being more than considerate in allowing him to move out 3 months early and taking on the burden of the full rent yourself.
As an aside, in addition to being an attorney, I have also been a landlord for over 26 years...