Thanks for your question, it's obviously a very difficult situation.
If she is jointly named on the land registry title to the property then you son cannot simply change the locks, whilst jointly named each party retains a right of occupation and access which neither can deny the other unless there is a court order to the contrary following civil or criminal court proceedings.
If he changed the locks she could simply change them back and probably claim the money from him.
If he wishes to take over the property solely then he would need to receive a mortgage offer in his sole name and gain her consent to the transfer of the title. He cannot transfer ownership without the consent of the lender or her consent.
She would be entiteld to ask for payment for her share of the equity in the property. If she did not agree then he could apply for an order for sale of the property in the county court. Their primary consideration will be the welfare of the child (ie. to see that the child is housed).
If both parents are named on the birth certificate for the child then they both have Parental Responsibility, this is slightly difference to "rights" over the chid. They need to at least work out what is to happen with the child in terms of residence and contact for the non-resident parent.
This in my view is the most important issue and they need to work out what they are going to do. If they cannot speak civilly then they should contact Resolution about mediation on, first, the issue of residence/contact and, secondly, what is to happen with the property:-
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