Is it legal in the state of Montana to fire an employee when they are on a workmans comp claim?

My mother works at an airport as a janitor. She was lifting a large box of tissues and injured her back in doing so. She has been out of work due to this injury for the past 4 months and has a workman's comp claim.The airport sent her a letter last week and said that they were terminating her employment because the doctors said she would be out for an undetermined amount of time and didn't know if she would be able to return to the work that she previously performed? Is this legal for the airport to fire her?