How to deal with a coauthor who introduces errors
I recommend two different approaches, depending on the other types of value contributed by your co-author:


If your co-author is not contributing technical value to the project at all, then you don't need to make a big issue of things: just simply do not invite them to collaborate with you on your next paper.
If your co-author is contributing in other areas, but inconsistently for the proofs, I would recommend using a good version control system (e.g., Subversion, mercurial, git).  Then you have an easy mechanism by which you can do explicit review of the changes that they introduce and adopt precisely the ones that you are positively comfortable with.