Oct 21, 2013
Maybe he has an alt account. I got one myself for regional purposes.
Do you have a cinema video of this match? It would help to have some evidence of this.
On some rare occasions, I've thrown a grenade and killed 2 people with it at the same time and if the score is like 49/45 in TDM, it'll end up being 51/45. This might be the case of how he got that additional kill.
EDIT - Solution already posted...