I find it hard to be sympathetic, personally.
You don't remove copyright notices when you copy code files from one repo to another.
This is seemly a clearly vindictive and deliberate action by the OP to remove credit for the files, due to whatever personal history:
> I would rather not have any mention or contribution of this person in our library, as they go against everything the Free Software movement stands for!
The GPL FAQ is unambiguous on this point: https://www.gnu.org/licenses/gpl-faq.en.html#IWantCredit
It's petty and seems quite dishonest to file a DMCA for something like this, and fixing it should give you a perfectly ok license to continue to use it.
...but just think, if you hadn't been petty and removed it in the first place, it wouldn't have been an issue at all.
To some degree, you reap what you sow.