If at the time of joining you have collected form 11 from the employee and in that he has declared that she was not a member of PF (with her previous employer) then your exclusion of the employee from PF for the reason that her salary (PF qualifying salary) exceeds Rs 15000 will be legally maintainable and she cannot demand PF at a later stage. But at the same time, if you had not collected a declaration in form 11 but at the same time the said employee is an existing member of PF, that is to say at the time of joining she had a PF account, then she can ask for PF even with retrospective effect from her date of joining. In such scenario, you will have to cover her with back date, date of joining your establishment, and pay both employee's contribution and employer's contribution together with interest and damages.
A 'consent letter' that you collected has no legal validity. At the same time, it can boomerang as an attempt to create negative covenant or an agreement against the law, which is not valid.
Now what you have to ask her is to produce the UAN. If she produces UAN, then she has the right to get enrolled to PF but on the other hand, if she does not submit it, you can deny her demand on the ground that an employee who is not an existing member of PF need not be covered if his/ her salary is above Rs 15000.