In terms of acquisition value, the codebase ranks dead last behind talent, product, engaged users, company brand, patents and just about everything else. Even very successful products are almost always rewritten when acquired by a larger company.
Depending on what your co-founder is walking away with, you may want to negotiate a better exit. If there is nothing else, then more than the code work on selling yourself to an interested company.
> If I were you, I would fight for ownership in whatever is next for your cofounder
Depends on whether the cofounder is using the current company's assets towards the next venture (and it seems like the answer is no, so there is really no basis for that ask).
Depending on what your co-founder is walking away with, you may want to negotiate a better exit. If there is nothing else, then more than the code work on selling yourself to an interested company.