DD has a flat head (plagiocephaly), so I am taking your rant a bit personally. Whilst I can fully understand your frustration about how your friend's baby, you might like to know a bit more about plagio.
The incidence has increased since the back-to-sleep campaign started, and yes, is probably associated with prolongued back lying. However, this is not the only factor. DD hardly ever went in a car seat (hated it!), was carried in a sling until 3 mths (I didn't have a pram for her), but yes, slept on her back. She however, had mild torticollis (stiff neck muscle), so whenever she was on her back, tended to turn her head one way. Latest estimates are that 7/10 babies with moderate to severe plagio have torticollis to some extent.
Indeed, I recently read research (from NZ, I think) has pipointed a correlation between preferential gaze (ie a tendancy to turn your head to one side) most stongly with babies who develop plagio/flat head - this was measured from 10 days/2 weeks - ie those children who were prone to the condition could be pinpointed at that age. Birth trauma (both slow & quick labours) have also been implicated, I believe. Obviously, what you are describing will exaserbate the problem.
I have just about stopped beating myself up that dd doesn't have the perfect round head that her brother has, because I listened to the h/v who told me it was normal/there was nothing I could do, whereas actually, she needed physio.
I really hope none of my 'friends' judge me as you are doing your friend.
I didn't really notice dd's head shape getting worse - I saw her everyday, and I think she's beautiful & perfect. Luckily, a KIND friend talked to me gently about it. I cried non-stop for weeks, but she did the right thing, and I have been able to take action to help dd, and her head shape is now almost back to perfect. Perhaps you could take a deep breath, and think how you would like to be told something like this.
Sorry, long post, but I suspect for obvious reasons.