Keep on convincing your husband with politeness and wisdom that talking to non-mahram women is unlawful in Islam. Only by being friend, they will not be mahram for each other, rather they are more emphatically asked to observe purdah since it makes involvement in sin more probable. Such a modernity which contradicts to the Islamic Shariah is unacceptable in Islam. Also, you should pray to Allah for your husband to guide him. Recite ya Lateef and ya Wadood after Isha 111 times with durood 11 times before and after. In-shaAllah, he will love you and will abandon caring for the other women.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband