What is the ruling of Islamic law concerning a wife who is involved in a suspicious friendship with another woman, and refuses to comply with her husband's request to stop this relationship, especially after the discovery of exchanged love messages between them?

Jealousy is commendable within its limits. If jealousy stemming from your wife's female friend causes suspicion, then you are permitted to forbid your wife from associating with her, and she must obey you. One should not harbor ill-will or investigate unless there is something that arouses suspicion. Part of guardianship necessitates care, along with seeking wisdom, good admonition, and supplication.