A few weeks ago, we (2 authors) published a hypothesis paper in the medical sciences.
Some days after the manuscript was published online, a researcher wrote me and asked for the full text. I did not know said researcher at that stage. We exchanged some emails back and forth about the content of the paper. I encouraged said researcher to publish his own ideas on that topic in a comparable hypothesis paper. Subsequently he wrote a draft and submitted it to the same journal. He informed me about that step and sent me the manuscript to have a look (after submission). Now the same journal where we published invited me to review the manuscript of the aforementioned researcher. We work in foreign countries and never published together before.
However, we had contact prior to his submission and he sent me some of his ideas.
Should I decline the review invitation in this case? I would love to review this piece because it is my main research topic. On the other hand, I feel it may be inappropriate that he sent me the piece after submitting it to a journal.