@sspencermo he files as a non -resident in MA and then files as a resident in MO/ if there is tax due to MA, there will be a credit on the MO tax return. Be sure to complete the MA tax return first.
How much does he expect to earn in MA? if less then $8000, there is no filing requirement, so he should fill out the forms to be 'exempt' from MA tax withholding
https://www.mass.gov/info-details/who-must-file-a-state-personal-income-tax-return
That all presumes he is not moving his residence to MA. what is a "post-bac research'? is this a summer job or is he moving there for a year with an undergraduate degree? if the latter is the case, why do you think he is still a MO resident? Look at the definition of a MA resident in the link below.
https://www.mass.gov/info-details/learn-about-legal-and-residency-status-in-massachusetts
