At a minimum, you need to confirm the taxpayer's status (Visa type, days present in US for the past three years, etc.), the (non-)existence of a tax treaty between the home country and the US, the type of TIN the taxpayer has, and what documentation (W-9, W8BEN, W8ECI, etc.) the taxpayer provided the brokerage. Once you have all that, the proper course may become apparent. There will be two things to consider: (1) Ideally, what would have been the correct method for the broker to report the income and for the taxpayer to determine tax liability and file a return, and (2) if the broker reporting differs from the ideal, what do you have to do to correct errors or get the tax reporting moved from the incorrect tax form to the correct one.
The snap answer would be that this should have been reported via 1042, 1042-S, but that is only the case if the broker had the appropriate withholding certificate from the taxpayer and tax treaties didn't specify some other treatment.
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