@Lalalax I can't add much to what people have said, except I agree that a new partner is unlikely to inherit over and above the deceased person's children, but is is possible that they could contest it, and be entitled to something, as people have said...
@mummagiles
Ignore the people who are saying your DF's partner would have no claim to the house. She could make a claim under the inheritance (provision for dependants) act as a dependant on the basis she has been maintained by your DF by his provision of accommodation. Her claim on the estate could amount to a sum equal to that which would be necessary to maintain her living standards as immediately prior to your DF's death. There was a case recently where a woman who had been estranged from her mother for decades successfully claimed against her mother's estate
Although I don't think the estranged daughter is a good example of what is going on with the OP, as she was the deceased woman's actual daughter.
To say because he is 'only 64' he doesn't need to make a will yet is one of the daftest things I have read on here in awhile! Sorry, but 64 is NOT young. You're a bloody pensioner FGS. Many people see their health start to deteriorate by their late 60's and early 70's, even when they have had good health for many year prior to that. You'd have to be delusional to think you 'don't need' a will at 64. Especially as the OP's mother died in her late 50's!!!
Re the issue the OP has, I think when someone gets a new partner/new spouse, any assets they had when they met should not go to the new partner/spouse in the event their death, unless they have been together 10 years or more.
I don't particularly think very highly of people getting very sniffy and assuming they have an absolute 'right' to their parents estate, but at the same time, I don't agree for a second that someone should be allowed to meet someone, and inherit everything they and their deceased spouse have worked hard for all their lives, potentially within MONTHS (or several years) of meeting them.
As has been said, this is not what the OP's mother would have wanted.
I know OP, that you said your dad 'wouldn't lie,' but for all you know, his new woman could be controlling him (no matter how nice she seems!) I have seen it happen. Not saying she is a gold digger, but I am very suspicious as to why your dad won't discuss it. You have every right to be worried and suspicious, but if your dad won't discuss it, there is little you can do.