Keep in mind that you are not a trained professional in these matters. So if you are not careful, you could put yourself and your employer in legal trouble. So your best policy would be to discuss your concerns and suspicions with the safety manager or chief pilot. At that point it is up to them to take action.
Do not go around talking to everyone about this pilot. Spreading rumors about someone doesn't help you personally or professionally. And remember that this is their livelihood also, so show them the same respect that you would for yourself.