That's a really rough situation to be in. The premade had to be out of newb island (as only one of them was even still in it; even one person out of newb island drags the entire premade out). The other team probbbably had most of the rest of the premade at pretty low Elo. Keep in mind that ridiculous amounts of wins can still mean low Elo, if he also has tons of losses. I'm not sure on the MM #s on this one, but there's some explanations.
TL;DR sorry for the bad game
. If you feel that the difference were so vast it should be considered a MM error, submit it to Zileas and maybe he can find something in there to tweak (weight the higher player more heavily...but then that means his 4 friends will feed, etc. I'm not sure).