Please, anyone, what is the ruling on a marriage where a husband has been in an adulterous relationship for many years without the knowledge of his wife. (he is muslim) He never married the other woman, but continues to provide money to her. Is her marriage to him still valid? should she forgive him, since he now SAYS he will stop (once he was found out) What should she do?

Subhanallah sister, bear in mind that there is a difference when a man marries a woman without letting his first wife know and a man who is commiting zina. Your husband committed zina. Isnt the punishment for that by the shariah to be stoned to death. I dont understand why he would not just marry her and save himself from the fire of hell- especially since he an afford it.
But whatever your decision is may Allah guide you and make it life easier on you. Dont forget Allah test those He loves.

Do Muslims still stone people?