It is probable that the freeholder of the commercial properties (now the developer) also possesses a right of access to the common parts. It is possible that the common parts are jointly owned by all the freeholders; it is also possible that the common parts are divided up so that everyone owns one 'bit' but everyone also owns a legal right of access to all of it (and potentially a legal right to share maintenance costs). In the first instance your friend would be best getting her title register out to see her own position is about ownership.
The way to find what rights the developer has over these common parts is to download a copy of the title register from the Land Registry site here. It costs £4. She may also want to title plan for another £4 (which shows the plan of the land the developer owns and will cross-refer any land in the common parts area that may be shown in the title register).
Some developers have a nasty habit of taking liberties when it comes to use of land, noise etc. during the development itself. The way to stop it is to challenge what they are doing (with regard to keeping materials on the land) or approaching the local planning authority (with regard to noise and dust) - they may be in contravention of conditions imposed during the build under the grnted planning consent.
http://www.landregist...ortal/Property_Search