This will depend on whether the individual has capacity to make a Will or not.  Just because a Deputy has been appointed to manage their financial affairs does not mean they will automatically also lack capacity to make a Will (testamentary capacity).  It is possible to lack capacity to manage your financial affairs but to have capacity to make or change your Will.
A formal assessment of capacity to make or change the Will would be required.  If that assessment confirms the individual has capacity to make / change a Will, they can do that for themselves in the same way any capacitated adult can.
If the capacity assessment confirms the individual lacks capacity to make / change their Will, an application to the Court of Protection would be required for a statutory Will.  The new Will could only then be made if approved by an Order of the Court.