He should contact an attorney in the county where the divorce took place and learn his rights. He should meet in person with the attorney and explain all of the facts to him or her. This is the best way of getting a comprehensive answer to your question.
The technical answer is "yes, it is legal" - she is not violating any laws that are regularly enforced (adultry is still illegal, but it has not been prosecuted in years)
From a family law aspect, it's clear that the mother is not acting in the best interests of the children. This is a reason to modify the parenting arrangement. A Petition to Modify Custody should be filed in the county where the children are currently living (the residence of the defendant) and perhaps even ask for a Guardian ad Litem. Judges do not like this sort of situation, and it may well also be in violation of the Final Judgment and Decree of divorce.
One side question, and I dont mean to sound offensive, but why did it take two years for your husband to find out? If he does not have regular contact with the kids or his ex, it may be difficult to prove that he is the better parent for the kids.
I wish you the best of luck. Should you have any further concerns, feel free to contact me email@example.com
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