Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Taking the nationality of any non-Muslim country for the purpose of business or comfortable life is not permissible. There are many legal prohibitions in this practice such as taking the pledge of allegiance, exposing oneself for the temptation, etc.
A person may trade in Muslim countries; one should prefer the countries that observes the religion, the forms of worship, the good values, and the good conduct.
Allah Says (interpretation of meaning): {… And whosoever fears Allâh and keeps his duty to Him, He will make a way for him to get out (from every difficulty). And He will provide him from (sources) he never could imagine …} [65: 2-3].
Therefore, your husband should avoid taking the nationality of a non-Muslim country. For more details, please read the Fatwa: 82552.
Allah knows best.