Lie of omission is still a lie.
If you and a friend are both competing for the same prize and you don't tell the friend while working behind is back, that friend isn't going to forgive you if you win. Especially if you've been "helping" him/her.
Ninofags are in such hard denial. It's like arguing that you aren't really cheating on your GF by getting sleeping with neighbor girl because your best friend was cheating on his GF by sleeping with half the campus. Yeah, one is "worse," but that doesn't make the other a lesser crime.