Montana will not discriminate. It wants to tax everything. My father (a Lacerte user) had a California client who won the CA lottery, and moved to Montana, even though he pointed out the CA winnings were now totally taxable to Montana.

"every nonresident who is a single person, and every nonresident who is a married person who does not elect or, as provided in ARM 42.15.321, is not allowed to elect, to file a joint return with a spouse, must file a return if the person has any Montana source income or loss and their gross income from all sources is more than $4,370"

It's that "all sources" that gets you. When you file separately, you are allowed to separate the income and expenses pretty much however you want, when the funds relate to commonly held accounts or activities. Then, you enter any specific and segregated funds per that person to whom it applies, such as, if only one gets Unemployment or a W2, that goes to that person and is not split or shared. If you pay a deductible expense from a joint account, it can be split.

Once you do all of this, does your person still not meet the MT filing requirement? Do they have a tax computed, even if this is all counted?