Is it permissible for a girl to reveal the secret of her friend who got married عرفيًا (customarily/unofficially) – without a guardian – while she is still a virgin, knowing that she was entrusted with this secret and made to swear an oath upon it?

If a woman marries secretly without a guardian (wali) or witnesses, her marriage is null and void. Whoever learns of this must advise her and explain the invalidity of her marriage. If she does not respond, then that person must inform someone who can prevent her from this wrongdoing. However, if the marriage was only without a guardian, the majority of scholars deem it invalid. But if she is following the opinion of those who permit it, then it is not obligatory to condemn her or reveal her secret. Rather, she should be advised and informed of Allah's ruling on the matter.