I’ve embedded various sidebar widgets and wotsits over the years on several different sites. There may have been a time when they were worthwhile in terms of visitor engagement, but the trend is to digest soundbites via social media and reader apps (like Zite, Feedly, Instapaper etc) that don’t necessarily show anything but the headline and body text of a post, this is particularly true on the mobile platform, of course).
It does depend on how many visitors he’s getting spread over the site rather than arriving from specific links on social media, the organic and search engine visitors. If it’s fewer than 100 or so, I wouldn’t expect to see anyone clicking anything. If it’s a few hundred a day then I’d assume a click rate on sidebar matter wouldn’t exceed 10. If he’s getting a couple of thousand uniques a day then I’d expect to see a few dozen, maybe more (see caveats above).
Based on his site’s Alexa rank, which is comparable to my Sciscoop.com site and its Google PR of 4/10 (Sciscoop is higher), then I’d assume 3 clickers is ballpark predicted give or take a handful.
Personally, I wouldn’t retain any third-party widgets that aren’t paying their way in terms of making the site stickier, more engaging or useful to readers, nor are paying their way with hard cash. They’re not even a visual distraction for most readers at this stage, but they can add to pageload times, which can be detrimental to a site’s search engine ranking in Google.
Another SEO point arises in terms of whether the widget is adding external dofollow links to the page and thus diluting the site’s pagerank without any benefit. I am toying with the idea of stripping back all my sites to the bare minimum, retaining social media icons, navigational and internal links and having nothing else but title and blog post content.