My friend is betraying her husband

In the name of Allah, praise be to Allah and peace be upon the Prophet (peace be upon him), his family, and his Companions.

May Allah reward you for caring for your friend, for one of the best qualities of a believer is to care for Allah's Religion and calling bad actions deniable.

Exert efforts to invoke Allah for her, continue to advise her, and tell her that the things which she thinks that lead to pleasure and happiness may turn her life upside down in a minute. It is enough that it is disobedience to Allah (Exalted be He) and contradiction to His Way, the matter is worse when she is married.

Remind her that Allah's Forbearance and Coverage for her should not make her deceived because the torment of Allah is coming. You mentioned that you are thinking of telling her mother but you are hesitant. My view is: Do not hesitate if her mother is an aware and responsible woman and can grasp and deal with the matter calmly.

Do not despair of repeating advice, perhaps Allah may guide her even if you are faced by some cruelty from her because the continuation of your relationship is better than interruption.

I suggest that you make her contact one of female callers to Allah or a female social reformer who may affect her positively by the will of Allah.

I ask Allah, the Generous and the Lord of the Great Throne, to guide her and make her content with her husband and detest sins to her, for He is the Guardian and the All-Able.