How was a legitimate marriage contract concluded for a woman who is still married to another man, given that she obtained a civil divorce in a non-Islamic country, and was not divorced Islamically, with official documents proving this?

It is not permissible to seek judgment from man-made laws unless there is a necessity. A divorce issued by man-made courts is not considered valid. If your son has not divorced his wife, she is still under his marital bond. Her marriage to another man is void and must be annulled. Anyone who initiated or assisted in this marriage, knowing that the woman was already married, has committed a grave sin. Both are liable to the prescribed punishment (hadd), and lineage is not established from such a union. The final decision on this matter rests with the Shar'i courts.