Mont. Code Ann. § 40-2-207

All work and labor performed by a married person for a person other than the married person's spouse and children must, unless there is a written agreement on the person's part to the contrary, be presumed to be performed on the person's separate account. This section does not affect the liability of earnings for debts incurred for necessary articles procured for the use and benefit of the married person, the person's spouse, or minor children, as established by 40-2-205, 40-2-206, 40-2-209, and 40-2-210.