Is it permissible for a person to sit with his disbelieving mother-in-law and be alone with her?

If the mother-in-law is the wife's mother, then she is permanently forbidden to her daughter's husband, whether she is Muslim or disbeliever. He is permitted to be in seclusion with her unless there is a fear of fitna (temptation/strife). If he fears fitna, then seclusion with her becomes forbidden.