Can somebody force disinheritance in order to have a debt repaid?

[fustbariclation]

Here's an interesting, and real, situation. A person, let's call him X, wants to force my mother-in-law to disinherit two of her children in favour of his ex-wife.

His ex-wife is not well and needs to be in a special home. He wants to pay for this special home - but only on condition that, when his ex-wife's mother dies, she leaves all the money to him to repay him for the money he has laid out for his ex-wife and to use the rest to continue to pay for her.

Now, I understand that my mother-in-law could, voluntarily leave all her money to just one of her three children on the grounds that she needs it most.

Can X force her to do this, though? If he did force her to make out a will in his favour, or his ex-wife's favour, entirely, would this be binding?

I'm asking this because my understanding is that anybody, including my mother-in-law, can write a new will that invalidates all previous ones any time before death.

So, if my mother-in-law agreed with X and made out a will in his favour, she could, the next day, make out a new will leaving the money in equal part to her three children and there'd be nothing that X could do about it.

Is that true?

This is all taking part in South Africa, if that makes a difference.