Maria59
Married persons property act applies to unmarried couples living together as well.
No it doesn't.
See here for the legal position.
If the girlfriend has paid towards the mortgage then she will have a claim on some of the equity, although as the OP's son will already have equity in it it will not be an equal share. If they agree that her contribution is towards bills and living expenses then she will, effectively, be a guest in his house and will not have a claim.
Suzy In principle I agree with you, but I would not want to live with a partner who could evict me with no capital behind me after what might be years of being together. OTOH, neither would I think it was fair to have someone move into my house and claim rights over it immediately. There are so many variables here - how long they have been together, how much of the house has been paid off and so on, that it is important that both parties understand the parameters of their arrangement, and that they both agree that they are fair.