I have been invovled in many games where one of my teammates fed their respective lane very hard, while the other lanes are staying even with the enemy and we end up losing a good percentage of the time. Bringing all lanes and jungle into the equation, what can we do to comeback from this without the game snowballing out of control?
Just recently TSM was counterpicked hard on top lane, and Dyrus fed 2 kills early as WW vs Olaf, and he was behind double digits in cs. His lane was done. Jungler never came to help because there was a high risk of being double killed by the Olaf. The way they came back was by completely giving up on that lane. Dyrus roamed to help pick up objectives like towers and dragons.. and he ganked mid and bot lanes. When he was in lane, he didn't farm outside of the tower, often giving up large quantities of cs.
So that's some advice for you if you're feeding. However.. if someone else is feeding, you're best bet is to make sure the other lanes win harder. Also, be POSITIVE towards the feeder. Perhaps they had a bad start but they're actually pretty good. You'll never know if you or your team degrades them so much that they start to focus more on yelling back than coming back.
The easiest way is to farm, farm, farm, and then farm some more; don't only farm your lane, also farm available jungle creeps as well as lanes that are left empty due to a death or something similar. By doing this, you help close the gap between you and your enemy.
Another step to take is focusing on stacking whatever counters the fed champion's damage. If an AP is fed, stack health and magic resist. If a bruiser is fed, stack health and whatever resist is most effective against their damage (magic resist for something like cho'gath; armor for a champion like gangplank; if they do hybrid damage like udyr or irelia, then stacking mostly health is a really good option).
Another option is to go all out to gank them really hard before they can snowball properly. In the case of mid lane, send your top laner and your support as well as your jungler to simply shut them down. For top lane, having your jungler camp the lane and your mid lane roam top as well can potentially undo some of the damage. Bot lane is harder, but still doable since top lane can gank through lane after returning to base with mid and jungle converging as well.
Shifting playstyle can also help overcome this problem. By focusing heavily on objective play, such as taking dragon and towers, you can keep the fed champion out of the picture by making sure to do things when they are either in base shopping, or trying to gank another lane. Alternatively, simply rotating your top lane middle and your mid lane top can occasionally help when the problem is based on bad matchup more than anything else.
One last measure against a fed champion is freezing the lane and hiding under turret. To do this, you want to wait until the minions are pushing toward your tower, collect them and stop them from being hit by it; usually by tanking their damage yourself. Be careful not to expose yourself to the risk of getting dived by doing this, but if you can manage the lane properly, this can effectively deny your opponent gold.
Try to focus them more and shut them down, ganking them often. I'd say one of the hardest champions to stop from snowballing would be riven because she can get so far ahead with just little pushes. Ward up for map control. It is often hard to carry a team if only one person is fed. If the rest of their team is derping and your team as good teamwork you should be fine.
Farm a lot and make sure the map is well, well warded. If you know they're in your jungle, ward your jungle, even if you're an AP carry and you're saving up. I cannot count the number of times we've killed a fed champion as a team who got caught out of position without their team's help. Make sure your entire team knows to hit them first, before the fight breaks out, and baiting them out from their team will make it easier to single them out and kill them quickly.
If you have a stun or slow, try to use it on the other, unfed champion to separate the fed one from the pack. Then have your teammates come in. If you can move the enemy into your turret or team, all the better. If you both have mid health, sit near your turret, and wait for them to take a couple turret shots before going in. (Not solo). Also, get the tiamat or ravenous hydra to keep those pesky turret-shot-blocking minions out of the equation. All in all, just get someone or something else to help you.