Is it permissible for my husband to talk to a strange woman over the phone in order to keep her away from his married friend who has children?

It is permissible for a man to speak with a non-mahram woman when there is a need, provided certain conditions are met: that there is no softness in speech from the woman, that there is no gratification from the man, and that the conversation is permissible. The conversation should be limited to the necessity to prevent any means (to sin).

Therefore, there is no harm in a man speaking with a woman during buying and selling, or seeking a fatwa, or for medical treatment, and the like, as long as these controls are observed.

As for a husband speaking with a woman and advising her to stay away from his friend, the benefits and harms should be considered. His contact with her might entail a benefit or a harm, and there might not be a need for it. This can be circumvented by an anonymous message containing advice and reminder. This varies depending on the circumstances of the husband and the woman.

The safest approach is for him not to engage in direct contact with her, especially if the woman does not care about forming relationships with men.

The husband should advise his friend to stay away from that woman and remind him of Allah the Almighty.