Factitious disorder by proxy or Munchausen Syndrome by Proxy (MSbP), are insidious disorders in which injury is deliberately and gradually inflicted upon a person usually for gaining attention or some other benefit.
In legal precedents in the U.K, Munchausen Syndrome By Proxy does not exist as a medico-legal entity, so they can only prosecute someone on the basis of harm to another where this is evident (abuse/neglect etc). You cannot be be prosecuted for having MSbP.
The Q doesn�t really say why she considered he might leave her, so we must surnmise that she had some form of undetected psychological or personality disorder for many years prior. It seems odd though in our current climate that this child�s condition could go relatively unchallenged/untreated for so long without further tests or physiotherapy.