Unfortunately it is. However questionable it may be, people that aren't using a hacker client that are teamed with someone who is, is allowed. This is because it would be a very unfair ban for these few reasons.
1: We have no clue whether they just teamed in the lobby and the team-mate had no clue that he was using a hacked client, due to lack of knowledge.
2: They could be just internet friends on a first name basis and may believe the player is just really good and the hacker hasn't admitted to his friend he is using a hacked client.
3: The player may be teaming with the hacker and may have knowledge of him using a hacked client, but the fact remains that it would be unfair to ban someone for teaming with a rule breaker since he isn't breaking any rules himself (We wouldn't ban someone for teaming with a player that was perm'd for abuse, modded client, etc).
However one thing I do not agree with is people using hacked accounts to boost the stats of another player. Recently I have seen this a lot of this on AU and I kid you not I was just going to forward the idea to Senior Staff before you made this thread. I believe in that certain scenario it is worthy of a ban (but there isn't a rule against it yet) but otherwise, not to try and encourage people to play with hackers, is technically not against the rules. But if your looking for a good reputation in the community I would recommend against that.