Counter Rengar is very easy and Now I'll help you!
Then, If Rengar is really fed, obviously he behaves like every assassin fed, destroys everything, but unlike Leblanc, Kata's animations ... etc, Rengar has much counterplay now. The only thing is that you have to work as a team: The ADC must never isolate, but you should aim to do teamfight, when you see the Ult of rengar, the whole team must retreat, the whole team, including the topper that like a junkie was launched towards the adc enemy, and while you're backing use a pink. If you see him, engage and he will explode( Excellent topper for this is Moakai). If he is coming from a different direction than where is the pink, no problem if you have a support awake: an exhaust, a heal, a slow stop rengar( for mage just zhonyas and a little 'hp anyway). If rengar is so fed as to kill the adc however, then just an defensive object to resist to rengar's burst and kill him with a little 'kite
https://www.youtube.com/watch?v=Qlu46PhfZKw
look at 51 min...XD
There was also another video where was shown the strategy of retreat, but I don't find it, sorry: P