@sjrcpaThanks for flagging me.

@abofficeIf your client's aunt is a green card holder, the pension to the extent taxable under normal rules would be subject to US tax and her aunt may claim a credit on the Mexican return for US taxes paid.  Otherwise, the pension is taxable only to Mexico, where she is a tax resident, based on Article 19(1)(a) and (2) but she must submit W8-BEN to claim the treaty exemption.  In the event it should have been treaty exempt but no W8-BEN was filed before the distribution, a 1040-NR may then be filed for a refund claim.