Is reusing old code for a new assignment considered self plagiarism? How to protect yourself if you consider it to be, and a group partner does not?
What should you do?  You should ask the professor politely and without imposing your pre-judgement on whether this is or is not prohibited by the rules of the class.  You are not in a position to make this judgement.  The only person who can give a definitive ruling is the professor who set the rules.

I recommend an approach like:


  My teammate has previously created code that would be very applicable to this project.  Can we use that code, or does it count as an external library, and we should instead write it again as an exercise?


While you are waiting for a response, I would recommend proceeding in two ways in parallel:


You implement a new version of the library from scratch, while
Your partner builds upon the existing library.


That way, you get the educational value of building the library, rather than using the pre-existing library, and your partner can push on ahead without either of you being stalled while waiting for the result.