Any number of people can obtain a TIN:
Nonresident alien required to get ITIN to claim tax treaty benefit
Nonresident alien filing a U.S. tax return
U.S. resident alien (based on days present in the United States) filing a U.S. tax return
Dependent of U.S. citizen/resident alien
Spouse of U.S. citizen/resident alien
Nonresident alien student, professor, or researcher filing a U.S. tax return or claiming an exception
Dependent/spouse of a nonresident alien holding a U.S. visa
to name a few (straight from the W-7 form).
More information is needed about the situation. I suspect they may be a nonresident alien of some sort, so their legal classification would need to be known.
A taxpayer identification number must be furnished on all returns, statements, and other tax-related documents and must be given upon request to any other person who must include it on a return or statement. For most individuals this is a Social Security Number, or SSN. If you are a foreign person who does not have and is not eligible to get a SSN you must use an Individual Taxpayer Identification Number, or ITIN.