You could leave the repository as it is and export this lib using:
git filter-branch --subdirectory-filer lib2
git help filer-branch
says:
--subdirectory-filter <directory>
Only look at the history which touches the given subdirectory.
The result will contain that directory (and only that) as its project root.
Developers of your project will not notice any change, because there is no change. - However updating that exported library would require you to run another filter-branch.