@deszcz_pyr Still not a problem with the rating system. It's impossible to account for the fact that you only play together in any robust rating system. If I was playing LoL for instance and was somehow playing on a team of all high diamond players and was a silver player myself, I doubt they'd have much issue carrying me to plat or whatever. Same goes with literally any other game ever that has team game rating systems. FAF doesn't even use ELO, it uses trueskill https://wiki.faforever.com/en/Rating-System
If you want him to quickly get to his true rating and stop gaining/losing more points per game than you guys then get him to play custom ranked 1v1s. What you're describing is not in any way an issue with the rating system, and it's not actually a good idea in practice for the lowest rated person to get more points than the highest rated person, and afaik no major team game has a rating system that does that.
There's no possible way to truly know who contributed the most to a win to attribute points in such a manner. It can only be based on if it's a win or a loss and what your current rating and deviation are against the likelihood of the outcome.