How to test if the object is created correctly while keeping the class under test in isolation?

It is often told that writing unit tests one must test only a single class and mock all the collaborators. I am trying to learn TDD to make my code design better and now I am stuck with a situation where this rule should be broken. Or shouldn't it?

An example: class under test has a method that gets a Person, creates an Employee based on the Person and returns the Employee.

public class EmployeeManager {

    private DataMiner dataMiner;

    public Employee getCoolestEmployee() {
        Person dankestPerson = dataMiner.getDankestPerson();
        Employee employee = new Employee();
        employee.setName(dankestPerson.getName() + "bug in my code");
        return employee;
    }

    ...
}

Should Employee be considered a collaborator? If not, why not? If yes, how do I properly test that 'Employee' is created correctly?

Here is the test I have in mind (using JUnit and Mockito):

@Test
public void coolestEmployeeShouldHaveDankestPersonsName() {
    when(dataMinerMock.getDankestPerson()).thenReturn(dankPersonMock);
    when(dankPersonMock.getName()).thenReturn("John Doe");

    Employee coolestEmployee = employeeManager.getCoolestEmployee();
    assertEquals("John Doe", coolestEmployee.getName());
}

As you see, I have to use getName() method of a class that is not under Test.

One possible solution that comes to mind is to extract the task of transforming Persons into Employees to a new method of Employee class, something like

public Employee createFromPerson(Person person);

Am I overthinking the problem? What is the correct way?