Thanks for your enquiry.
The top Flat owner only has the right to occupy the area comprised within their Lease. On the basis that their Lease does not include the roof space, then she does need to obtain the consent of all the Freeholders before extending. Indeed, as Freeholders, all of you are entitled to a sum if any Leaseholder wishes to extend the area of land comprised in the appropriate Lease.
So, IF you were to consent, you would be entitled to charge her 2/3 of the market value of the new Flat and in turn, she would need to obtain a new Lease to include this additional land.
You would need to speak to a specialist Surveyor on this subject if matters do proceed.
I hope this assists and answers your question.