Selected answers from the Dumb SEO Questions G+ community.
Dido Grigorov: If it`s just one article - no problem! But keep in mind that Google might rank Reddit better than his website, which is absolutely normal due to many factors :) So I would recommend to repurpose the content in a different way, to extend it a little bit, make it a presentation, video or anything else, depending on the resources he has :) Could be also another completely different, unique and useful article based on the first one with an external link to Reddit for better UX :)
Rob Woods: yes, point back to the original with the canonical link element
Roger Montti: Any PageRank accrued to the client page with copied content may be credited by Google to the Reddit page with the original content. Any links your client gets will credit Reddit, not the client.
So that`s a win-win... for Reddit. ;)