I am writing a paper with a co-author. I spent a considerable amount of time (several months) writing a first draft of the paper. I then sent it to co-author asking them to revise the draft and add their contribution in specified sections.

The co-author returned the draft within a few days. The required parts were added and the writing was heavily edited. The style of the new document looked similarly to a few other documents that I knew were produced by ChatGPT. I asked the co-author and they admitted (quite proudly) that they used ChatGPT to assist with writing and editing of the manuscript.

I was not happy with the resulted state of the document. There were a few obvious errors, which I pointed to the co-author. But also, the whole document seemed a bit weird to read. I could not really define it, but reading it gave away a feeling that the document was AI-generated. It bothered me, that there could be other mistakes in the document are not immediately obvious, and that it reduces the chances for the paper to pass through the review process.

I shared my concerns with the co-author. They agreed to correct the mistakes I noticed. They claimed that they checked the document after it was generated, but a small number of issues possibly remained. They dismissed my argument that the writing is unnatural and weird to read, and requested to point out specific line-by-line errors, which should be corrected. They suggested that I could proofread the document and edit it again before submission, since I have concerns about its quality. 

I feel that co-author did not put sufficient effort in their writing, and leans on ChatGPT, my work as a co-author, and work of peer reviewers instead. This leaves a bitter taste in my mouth, but I do not know if this behaviour is unethical or wrong in some other way which I can't quite describe. 

Are my concerns valid and how is it best to communicate them to the co-author?