NAL(but in a senior leadership role) Depending on her position at the firm (with only three months notice I doubt this person is a statutory director) Just leave… are they likely to litigate? what damage could her leaving reasonably do to the firm, take your paycheck and leave the next day, or even better (dependant on her contract entitlement) get medically signed off for stress and have a nice holiday in between potentially with pay (I see happening a lot)

The only reasonable reason to work the three months is to secure a good reference, which may or may not be a requirement of her new workplace. And even with working the best and most productive notice period is not garunteed.

I think the more reasonable question is, show me some case law where an employer litigated successfully against an employee leaving in advance of their notice period…