If she wants to go down that road he will be entitled to half her shares, half the equity in the house and maybe a portion of her pension. Nobody will come out a winner.
Your son should consider offering to let her keep all of her shares, her pension, her car. In return she would allow him to buy her out of the house as had previously been agreed; he keep his business and his cars.
That's not only sensible, it will be by far the cheapest and most cost effective way for them to manage the division of their assets.
I assume there are no children and neither are close to pension age.