Is it obligatory to inform a friend that his wife contacted the questioner, with the potential for divorce, or should the matter be concealed and left for him to discover on his own, with the risk of losing the friendship?

If the woman contacts you again, you must advise her and warn her of the consequences of her actions, and threaten to inform her husband if she does not repent. If this does not work, then you should inform her husband so that he may rectify her, thereby absolving yourself of responsibility, as Sheikh Ibn Uthaymeen stipulated.