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 several 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. I was concerned with the possibility that there could be other mistakes in the document, which are not immediately obvious, and that it could reduce the chances for the paper to pass through the peer 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?

My first language is not English, and the same for my co-author. I appreciate that ChatGPT can correct errors in the use of English and improve grammar. I am concerned that it might introduce factual mistakes, change slight variations of meaning, or generally make the text unpleasant to read (although as a non-English speaker I find it difficult to explain why it happens).