I've seen writers bitterly complain when the friend of a published novelist gets a book deal -- certainly they're just riding coattails. There might be a correlation here, perhaps even a causality, but I sincerely doubt it's the one complainers are thinking of when they shake their fists.
Let's assume that the published author and friend are both in a writing group together (I can name several incidences like this, so I think that's likely). People bring different strengths to a writing group. Something I've noticed about a good writing group is that people have different strengths. Someone might have a killer eye for plot holes. Someone might tune into how well the emotions are working, or the pacing, or a dozen different things.
All these things work together for the good of the author and the manuscript, who goes out and sells it. Huzzah! A breakthrough.
Would it really be surprising then, that a second person from the same writing group also get a contract? Maybe the friendship helps them wiggle a toe in the door, but I'm convinced manuscripts stand or fall on their own merit. This second author is critiquing and writing regularly in a good group. He or she is likely also doing other things (reading, going to conferences, sending out manuscripts). I doubt having supporting writing friends hurts, either.
Which brings us to our last Critique Secret: Good critique groups help each other succeed. Hopefully after this series, that doesn't sound like a secret.
Do I think it's coincidence that there are a number of writer's friends who get published? Of course not. But I doubt there's some secret ring of agents and editors laughing maniacally in the corner, either (unless they're currently watching Megamind, which was excellent).
There's a lot more to say about critiquing (I'm sure I'll come back to it in the future; I love this topic), but for now, here's the link to Critters, an excellent online critiquing workshop. Click on "Workshops" on the far right to find genres other than SF/F/H. If you don't have a writing group, dive into this one! The water's great.