This is really sad that we are hearing such cases very often nowadays
i would ask her to keep patience and do sabr. and not to change her behavior to him. Most of the time, women out of anger and stress, start nagging n keep doubting on him for everything he does. This will start taking him away fr om her
Tell her to Think abt herself n d way she treats her husband. It may be that c has changed in the way c was treating him, without realizing. Be the best wife c can to him, and the best help, after Allaah. Stand be his side during this trial and be his support, after Allaah.
Also advise her to weigh up her husband’s good points and bad points, and do not forget his good qualities n his kind treatment to her, because that will motivate her to try to bring him back to the right path in his religion and in his relationship with his Lord, first of all, then in his relationship with her
Also ask her that it may possible that her husband has no intention to keep or make any relation with that lady. She calls n text her dsnt prv that her hsband is having affair wid her. tell her to give positive signs to him without leting him know that she is afraid of him going to another lady.
Doing this is not easy tho. v know how possessive women r. but she has to keep trust on Allah. tell her to do more n more dhikra and ask help from Allah.
May Allaah make things easy for her, and make her husband happy with your obedience and make her happy with his good treatment. And Allaah knows best.