fed riven or the adc???!!!!

ok 2 games we had a riven who was fed early.....she was 5-0 and then just started killing everyone and it was 21-12. 12 us and 21 them and riven was 18-0(that riven tho). so I said to the team that we r ok. if we just focus riven and kill her their whole team would be useless. but the team said no we need to focus the adc( vayne - 1-3-5) and not to go close to riven. and we lost the game as riven butchered us all and ended up with a perfect score of 24-0. so I think if only one guy on the other team is fed we should focus him not the adc or the assassin. ik they r important but they were useless at that time as no one else was fed other than riven. so what do u guys think.!!!!!????