She cannot simply say that this was an inheritance and have it back. What she would have to prove is that the inheritance wasn’t meant to be and have not become a matrimonial asset and if she paid the mortgage off because at that time, the marriage was going okay, the fact that it’s has subsequently gone pear-shaped is, to a large extent, not relevant.
She may be able to claim a larger share but it’s not going to be the whole of her inheritance in all probability. In fact, you can also argue that you made all the other repayments over the years and hence, one offsets the other.
The reason for the breakup is not relevant.
Can I clarify anything for you?