It depends entirely on the lease. If the roof and space is not included in the lease then they are intruding into and altering property which you jointly own.
The freeholders, you and FFF, therefore have an asset which has a vlaue which the freeholders are entitled to a vlaue form FFF as a leaseholder.
If however the roof and area in within their lease then likely it is imply a matter of consent and adjusting the service charge percentages for any shared costs or insurance.
As to consent I would want to see that the builder is reputable experienced and preferably qualified and in order to grant consent, that the freeholders, you and FFF appoint a charted surveyor and or engineer to approve the works, and that FFF not just the builder has adequate insurance in place.
They will not be able to mortgage or sell without that consent.
That said how nice it will be living in a building where ultimately you could end up hating and beholden to each other
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com