Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister, thank you for your question and for your concern about your friend’s affairs.
You should not tell your friend about it as this may destroy their relation. Instead, you should try to pass an advice to her husband trying to convince him to quit this practice.
Responding to your question, Dr. Muhammad Salama, PhD in Islamic Studies in English and an Assistant Professor at the Faculty of Islamic Science, Al-Madinah International University, states:
NO sister, we are exhorted to conceal the evil affairs of our fellow Muslims particularly if revealing such affairs will cause more problems. The Prophet (peace and blessings be upon him) said, “He who conceals the [evil] affairs of a fellow Muslim, Allah will conceal his [evil] affairs in this world and in the Hereafter.” (Al-Bukhari and Muslim)
Telling your friend about her husband’s cheating on her will cause family troubles and disputes, while this could be a passing whim that her husband will soon be recovered from. Many men get involved in such practices for a while and then give them up.
It will be better if you can find a way (through your husband, or brother, for example) to admonish and advise her husband to quit what he is doing to maintain his marital relations.
Allah Almighty knows best.