I agree it is a form of cheating. I have just finished a game on hardcore where my "Teammate" tried to destroy my AGR and another "Teammate" destroyed our Sentry Gun. Luckily we still won but it really could have made the difference between winning and losing.
Report him for what? if he had have eliminated all of the enemy you would have won and your rushing would have been a waste and he could have reported you for not getting a kill and not helping him win..
If he doesn't know anyone on the enemy team then letting them win isn't cheating...
see how silly your question sounds?