Okay, from a week or two of talking about what makes rank move and why playing with friends doesn't work well necessarily, I've finally realized why playing with friends outside of your rank doesn't work.
Friend one placed 50, as did you. You are supposed to be a 20 because you're awful, but you got carried and placed well enough. Your friend is actually a pro (lets go with seagull) and is supposed to be 80+. You play any given 60 matches and go 30-30. However, because you are at an inflated rank, your wins are only 40% of a rank or so, and your losses are a full rank. You will end at roughly 32 and your friend will end at roughly 68, even though you both played the exact same matches. By this point, you will be placing with people so good you cannot hold your own at all and will hate this game more, dragging your friends rank down slightly every match unless he can carry you harder.
This is why the game is unfriendly to a variety of skill levels playing together- it puts a crazy wall up that makes it increasingly hard to play outside your skill level or to be carried by others. It's better for a 50 to play within his level to get better than for a 70 to carry him, since the game will push back and throw him right back down to 50 after awhile.
From this we can also take away that a win/loss is solely a trigger as to whether or not the game will check if you're outplaying your rank or need to be thrown back a bit to learn how to play at your current rank. A win/loss doesn't actually affect your rank that much other than the 40%, as it's just trying to throw you into your generalized skill range. This is why so many people are stuck between 45 and 55, because the game doesn't believe they truly are top 15% in the world, and as such, it throws them right back to where they were, unless you go on a great win streak by chance and get out of it for a bit. It does not care if you went 2-1, you can still end as a 25, and someone with a 33% win rate can still hypothetically end as an 80. Improvements are more important than winning every match for ranking up, assuming you aren't running into the varied group rank issue described above.