Someone close to me was at a restaurant recently and she happened to see the boyfriend of her non-muslim friend with another women. The person I'm close with saw the boyfriend and the other women be somewhat intimate and this lead her to believe that the boyfriend may be cheating. So she was wondering whether it would be permissible under those circumstances to inform her friend of what she saw, or is this a matter we should stay out of.

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There’s no obligation to inform but also no harm in doing so either, guess it depends on the level of friendship as gheebah is only of believers.