Peer review - discussing paper with the author
The benefits of peer review are first to detect and prevent fraud and ridiculous claims, and second to improve the clarity of the published record. So, in general terms, if your conversation furthers these objectives, it's a beautiful thing.

On the other hand, if formally reviewing the paper comes with a commitment to maintain confidentiality, you will want to reconsider, and think carefully about how you back out of either the conversation or the formal review.

Confidentiality has benefits, like allowing reviewers to be critically honest without fear of retribution. It also has problems, like the inability to have an efficient conversation about honest disagreements and misunderstandings. This handicap is why some journals (along with the preprint system) are starting to allow for conversation without the blinds of confidentiality. If your journal-specific reviewer expectations do not prevent an open conversation, it may be the best approach.