I would have said that you don't 'get notice' of TUPE. TUPE is the transferring of your existing rights as an employee to employment in another company, when your old company is sold or taken over or agrees a transfer of employees.
I'm not sure I understand the situation you're describing, but I think you're saying that the employee will be kept on (from September to December), but in a different role to that pre-September? If so, then she could still be made redundant on the basis that both her roles no longer exist. Or transferred to the other company, which is when TUPE would apply.
As an aside, it seems a bit to transfer all these employees to the other company solely to make them redundant.