You need to research how that source of the pension is treated by MA:

https://www.mass.gov/info-details/learn-about-tax-treatment-of-pensions-in-massachusetts#nonresident...

You've made a couple of assumptions that the 1099-R is wrong (it's reality) and that this isn't taxable in MA. If the 1099-R has the right address for your taxpayer, then someone knows this person no longer lives in MA, and that's not likely the issue.