Is it permissible for a man to intervene in women's mockery of his wife (due to her non-Sunni lineage), and is it permissible for him to keep her away from them or sever ties with them?

If women mock a young man and his wife, they are doing wrong by that, because mockery is forbidden in Islamic law. Allah Almighty says: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them. And do not insult one another and do not call each other by [offensive] nicknames. Wretched is the name of disobedience after [one's] faith. And whoever does not repent - then it is those who are the wrongdoers."

Among the best ways to deal with mockery are: 1. Turning away from the one who does it. 2. Enduring it patiently, seeking Allah's pleasure. 3. Seeking Allah's help against them and occupying oneself with His remembrance. Allah Almighty says: "Indeed, We are sufficient for you against the mockers."

And if a Muslim needs to respond to the mocker, there is no harm in that, provided that the legitimate Islamic guidelines are observed. If these women are non-mahram, the young man should not have any relationship with them, and he may keep his wife away from them if he fears they might corrupt her.