Question about refinancing a mortgage and the tax implications?

My future son-in-law is trying to refinance his home mortgage. The man handling the refinance suggested that he might get a lower interest rate if he included my daughter's income and assets. They are unmarried, living together in Minnesota and expecting their first child in December. There are two parts to... show more My future son-in-law is trying to refinance his home mortgage. The man handling the refinance suggested that he might get a lower interest rate if he included my daughter's income and assets. They are unmarried, living together in Minnesota and expecting their first child in December. There are two parts to this question: Part One - Is this true about the possible lower interest rate and a good idea? and Part Two - Would they have to file taxes together from that point on? I would appreciate any advice and comments.
Update: Forget about Part Two of my question - I already know the answer to that, I don't know where my mind was. Appreciate answers/advice on part one. Thanks
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