I am currently attending a week-long conference. A few weeks ago, a quite respected professor in my field posted a paper on an online preprint server, making a series of claims, which are quite linked to my own research. In the beginning of the conference I approached him, asking if we could discuss his latest paper. He said yes, but asked if we could maybe wait a couple of days, since he was swamped with tasks during the beginning of the conference.
In the evening, I am discussing this paper with a couple of colleagues over beer. I am quite critical about some of the claims, and I voice my criticism. After the discussion, one of my colleagues pulls me aside. He says that he is currently reviewing the paper for a journal, and since I obviously have strong criticisms, he would like to suggest to the editor that I join the review panel, if I have no objections. I agree, he contacts the editor, and I am now reviewing the paper.
My question is now: Can I still discuss the claims of the paper with the author as we initially agreed upon?
(By the way: I know that my colleague overstepped the line by revealing himself as a reviewer. This is not part of my question.)