Absent a marital relationship, and absent either a will or joint ownership of the property, no part of the house will pass to your father's partner.

There are a couple complex scenarios, however, whereby your father's partner may be able to make a claim against his estate for contributions to the property, or perhaps for her reliance on his continued promise to transfer the home. To avoid this, it would be better for your father and his partner to agree on the ownership of this property during their lives.

She can want whatever she wants. If her name is not on the deed, she may have other remedies, but no direct interest.

To answer your question is fact dependent.

The cases allowing some restitution from an estate make it very difficult to actually obtain. Moreover, there must ghenerally be evidence of the value of services rendered. Girlfriend can make trouble and litigaion costs, but faces an uphill battle in any claim for an interest in your father's estate (absent his choosing too give her something, that is). If your father does not wish to provide her with an interest in the house, she is unlikely to get one absent very unusual circumstances. I have recently worked on such a matter in the appellate context, and can say with certainty that the girlfriend would have to show unique circumstances, including some implied promise by your father.

That being said, your father should make some decisions and get legal assistance. "An ounce of prevention is worth a pound of cure."

If your father has no desire to provide her with an interest in his estate, he should first and foremost not agree to have her placed on the deed at all. That is uniquely his power.

He should then also write a will in which he makes clear what he wants the girlfriend to get: that and no more. Depending on his age, your father may indeed wish to consult with an elder law specialist whose overall estate plan would be designed to maximize his ability to get care as needed, while protecting his assets to the degree possible under Massachusetts and Federal law. Your father should be made well aware that granting the girlfriend a right in the property without clear and definable consideration may in fact -disqualify- him from being provided with nursing home or other care with Medicaid support.

I wish you and your father the best of luck in sorting all of this out.