If she is a resident alien, ALL world-wide income is subject to U.S. Income taxes.

If the pension is less than $3,600, she owes ZERO tax and has NO requirement to file.

Further, she is entitled to a $3,600 personal exemption and a $5,500 standard deduction, so if the total pension is LESS than $9,100 for the year, she will owe ZERO federal income tax, though she WILL have to file a tax return.