Realized a co-author inserted plagiarism into our paper 8 years ago - what to do now?
Move on. In the abstract, of course, we should care about these things but, in reality, a paper that has been out there for 8-10 years has passed the statute of limitations. It is exceedingly unlikely that anyone will go back and check papers that old -- indeed, even if someone finds textual overlap with other sources, it will require quite a lot of work and human intervention to determine who copied from whom.

The point for you to take away is this: You learned a lesson to be more careful in the future. That's a good thing from your perspective, and it will make sure that you won't get into a potentially embarrassing situation again in the future. In the meantime, relax.