ANSWER:

If he does this, he must be flogged 80 lashes for slandering her like this unless he can provide 4 male witnesses to prove his accusations or get a confession of her in front of an Islamic judge or curse himself after swearing by Allah 4 times that she is an adulteress and 5th time that May Allah’s curse be upon him if he was lying. 

If a husband divorces his wife after finding out she was having an affair with someone else,is he sinful if he warns others about her ex-wife (warning other men not to marry her because she commits adultery,telling everyone the reason he divorced her etc.)