This person has a joint account in Interactive Brokers with his wife. The couple are not U.S citizens. Suppose the husband passes away suddenly without warning. Is the joint account subjected to U.S estate tax?
If the brokerage account holds US assets, such as the stock of US companies, then it may be taxable under some conditions. The rules are complex and depend on the nationality of the individuals, because the results may be affected by tax treaties between the United States and whatever country the person is from.