Is it permissible to bury a foreign man in the grave of a woman who passed away twenty years ago, and what should they do after the man has already been buried?

It is not permissible to bury two people in one grave except out of necessity, whether they are two men, two women, or a man and a woman, and regardless of whether they are mahram relatives or not, unless the first deceased has decomposed. It is forbidden to inter a deceased person with another, even if they are related, before the complete decomposition of the latter. The estimation of this is referred to those knowledgeable about the earth. If the burial has taken place and the body of the deceased has changed, the grave should not be exhumed.