Is the marriage valid, and what is your advice on the matter after the husband discovered that his wife was divorced and did not inform him of that until after the marriage?

Praise be to Allah who guided this woman to Islam through you.

The fact that the woman has been married and divorced is not a defect that warrants divorcing her. Rather, treat her kindly and strive to deepen the principles of Islam within her.

If the marriage was conducted under the guardianship of a disbeliever (father or otherwise), then it must be annulled immediately. Then, if you both wish, you may contract a new marriage, and let the woman's guardian be a Muslim relative if one is available. Otherwise, her guardianship belongs to the Muslim community in the country where you reside.