This would be a funciton of typesetting. The table of contents is indexed to the document as it is typeset, not dynamically throughout its existence. If you were to delete all the pages except for the table of contents, it would still refer to all the same pages.

If this is a necessary part of your workflow, then it sounds like a WYSIWYG tool like Scribus or InDesign is more appropriate (unfortunately).

The problem is that my document already is finished. First I could just deliver a PDF file. Now they want to edit it themselves. Or can I generate from my tex file something that has the meta information and can be edited in Scribus?

Nope. As Mojca mentioned, PDF does not account for this kind of thing. I mentioned those tools as a basis for constructing an entire document from scratch. They have automatic page referencing similar to Context, but not in a post-hoc fashion.

If they are only copy editing, I think you would be best served by exporting to xhtml. I generally write all my documents in Markdown and convert using Pandoc, so I'm not familiar with Context's xhtml capacities.

If they are doing layout.. Ouch.