Thanks for writing back -- good to hear from you. I will be glad to comment further, and I appreciate your taking the time to provide the additional clarification.
What you have provided is evidence in support of a finding of a common-law marriage. Ultimately, in Montana only a Court can officially make that determination. No one item of evidence is determinative, but at this point the best we can do is proceed as if the status of the marriage has been confirmed.
Turning to your question, namely concerning your daughter's rights, here is how Montana law works in that regard. A common-law marriage is a real marriage and requires a legal Dissolution of Marriage (divorce) to terminate the relationship. The most pressing need at this point is to secure temporary orders from the Court. This is in the form of a restraining order, later turned into an injunction, to maintain the status quo. This would prevent the man from disturbing your daughter's property and prevent him from having his mother come into the home and go through things.
The bottomline is that your daughter is in urgent need of conferring with a family law attorney just as soon as possible. The cheapest way for her to do so is through the State Bar of Montana's free, non-profit, Lawyer Referral and Information Service. She can start the confidential process by calling(###) ###-####Monday through Friday from 8:00 a.m. to 3:30 p.m. If she is unable to afford to meet with a private lawyer, she should contact Montana Legal Services by calling(###) ###-#### ***** wish that things were simpler, but the truth of the matter is that given the attitude of her husband, your daughter is going to need judicial intervention to protect her rights and interests here.
I hope this information is of some additional help and that all of this works out properly for your daughter in this matter.