What should you do if you know someone who is a accountant employed by a listed company on your national stock?

It seems that you have just heard confidential and price sensitive information which would if publicly known, probably affect the share price of both the acquiring and the acquisition company. If you or the employee of the company (the accountant) decided to act on this information by trading in the shares of either company, this would in most countries be classed as 'insider trading' and would be considered an illegal act and punishable. Your response should be to remind the accountant that this information is highly confidential and alert them that trading on this would be illegal and advise them against doing so.