You're making it sound like this is an easy thing to do. Find higher ranked friend, and bam, your elo rises.
When you duo, the combined MMR is significantly higher than the average of your two MMRs. So, if two Silver players duo, they're playing against Gold players.
If a high Plat player duos with a Silver, they're playing against low Plat players, not Gold or Silver players. Now, if your Plat friend is so great that he's able to consistently win games with the Silver teammate, one of two things are happening: the Silver player is not as bad as his MMR indicates and was on his way up anyway, or the Plat player is actually high Diamond, but he hasn't stopped rising.
In both cases, both players deserve to rise, and they both will. When a high league player is playing with a low league player and winning, they're both increasing their ratings. How come nobody ever calls it boosting for the higher ranked player? If it were so easy to win, then all higher league players would be looking for lower leaguers to match with and increase their own chances of winning.
Funny, they aren't doing that. Maybe it's because it's actually harder to win games that way than if you're playing only with people in your own range.