If it was thawed quickly, such as in hot water, or has been only out overnight, it should be refrozen immediately. Animals of this size begin and continue to decompose inside the snake's gut for a couple of days anyway, before the digestive juices take over, break through the stomach wall and effectively sterilise it. If it has been out for longer, or in a warm or hot place and has begun to smell, you should probably ditch it. Their digestive systems are not like ours - we chew our food and it is already being broken down by saliva before we swallow it, a snake swallow whole animals in one piece, so decomposition continues for a week or so inside the snake. (Ever smelled a regurgitated rat or rabbit???)
Jamie