She does not.I’m not sure if your Mum has to agree to it.
I've already posted
https://www.gov.uk/joint-property-ownership/change-from-joint-tenants-to-tenants-in-common .
which says:
Change from joint tenants to tenants in common
This is called ‘severance of joint tenancy’. You should apply for a ‘Form A restriction’.
You can make this change without the other owners’ agreement.
All dad has to do is serve notice of his intention.