i think he would have found it a hard choice but don't think he would have let him had the kiss in book 5 he actually checks to make sure sirius is alive in grimmauld place and advises him t stay there no one was around then he could have easily told him harry was in danger to get him killed and also in book 6/7 can't remember which dumbledore asks how many have you let die and he answers just those i cannot save i think snape he would have liked sirius to be in his debt by saving him