What is the advice regarding a colleague who jokingly deemed alcohol permissible, and was advised to go and ask about the ruling on his statement?

If your friend genuinely intended by "selling alcohol" that he would sell it, then he is sinful. But if he intended to permit its sale by law, then there is fear of disbelief for him, because some scholars consider this an act of making lawful what Allah has forbidden. However, if he meant that he considers alcohol permissible in itself, then he commits an act of disbelief thereby. But if he was joking, then joking should not involve uttering falsehoods and forbidden things. He must repent, and your disapproval of his statement is correct, and you will be rewarded for it.