Your father may be totally out of luck. If they were never married, he is not an heir and if the GF left hi nothing, he has no basis to contest the will.
If a probate is opened, he may have a claim against the estate, but he will need to prove how much he contributed to their living arrangement. He needs to visit with a probate attorney.
I do not think he could contest the will but he might(tough case) have a claim on the estate.
He should discuss with a local probate attorney that can advise him.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
If he can demonstrate that he loaned her money for living arrangements/support or improvements to the house, he could make a claim against the estate as a creditor.