Yeah this is the rating system working as intended. It won't be as accurate for your friend because you always play together and a team of coordinated 900s will probably beat a team of uncoordinated 1100s (idk for sure, just a guess) which will cause some rating inflation, but eventually it'll settle down for him. There's not a snowball's chance in hell your friend hits 2k unless he can actually play at least close to a 2k level if you guys are mostly playing 4v4 tmm. You already see his rating starting to level off in the graph.
You even said it yourself, he has an over 50% w/l ratio for the entire time his account has existed, why wouldn't he get more points for win than losing? Eventually you guys are going to drop below a 50% w/l and then things will change. You're not going to magically have a 1500 rating average on your team because of this one dude and beat a team of actual 1500s.
Edit: Plus this isn't even what this thread is about, though it's an easy enough mistake to make here I think