I don't understand why your MIL would like you to be friends with your hubby's ex. Part na siya ng past ni hubby mo, ikaw na yung present. So hindi na dapat pakialaman ni MIL kung maging friends man kayo ni ex o hinde.
Anyway, sabi nga nung ibang mommies, you don't have to become friends with anyone na sapilitan lang. Kung maging close kayo ni ex, that's good, lalo na nga kung may kids sila ni hubby. Otherwise, just be civil. Or kung talagang parang magsasabunutan kayo tuwing nagkikita kayo, try to avoid each other na lang as much as possible. Hehe.