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A Will can be made at any time before a person's death.
So, the fact that it was made a day before he died does not make it invalid in itself.
However, if your Mother's partner was not of sound mind/mentally capable of understanding the terms of the new Will when he signed it, your Mother would have grounds to dispute the validity of the Will. If he was of sound mind, I am afraid your Mother will not be able to dispute its validity.
On a side point, on the basis that the WIll is valid, your Mother may have a claim against her partner's Estate under the Inheritance (Provision for family and Dependents) Act 1975 if they were living together as man and wife and reasonable financial provision has not been left to your Mother under his Will.
Your Mother should take advice from a local Wills/Probate Solicitor in this regard.
I hope this assists and sets out the legal position to you.