It’s absolutely wrong.
First, when he gets to his boosted ELO he’s going to drag his teammates down. This isn’t fair to them.
Second, when he’s climbing with his boost buddy it’s not fair to his opponents because they have to contend with someone above their rank whos “slumming it” to boost his friend. Which is also called cheating.
Don’t carry your friends. Let the MMR and SR systems do their jobs.