This underscores the foolishness of men who trust others around their women, such as partners, sisters, or mothers. Mixing can ultimately lead to betrayal.
A friend of mine trusted his best friend deeply. That friend ended up sleeping with his wife, which devastated him. I often warned him that mixing men and women is risky. Ultimately, someone will find something appealing in the other, and this can gradually lead to cheating and betrayal. I don't care if he's your brother, father, cousin, best friend, colleague, boss, business partner, gym member, or teacher—sooner or later, he may lust after your woman and make a move. His intentions are high, especially if he believes there’s a good chance to succeed.
In my friend's case, his best friend kept visiting his house. My friend and his ex used to sit with this friend... Over many visits, she found him very funny. Eventually, he took her number to plan a surprise birthday party for her husband. After the party, they exchanged memes, and their relationship turned sexual, lasting ten months before my friend discovered it. The outcome was predictable.
Many men blame their wives or partners for cheating, which is understandable, but they should realize that their mistake- allowing another man near their woman- is what initiated it.