Well, in cases like those, you have to be really careful with choice of words. It really depends what kind of person they are how they respond, so it's not easy knowing what to say. As a general rule though, just be polite.
Don't say "Don't die" or "Stop dying" first of all. If they're having a bad game, that just makes it more frustrating. Especially if they're playing against someone like Fiora or Zed who keeps killing them under turret. I think the best response in general is just asking them to play safer until they get a gank. Although, know that that doesn't work all the time either, and some people will still get angry.
The best course of action though would be to actually try helping, although it's not always an easy task. to help top as a bot laner, it could be an idea to let the Support roam from lane if the ADC doesn't need the support at the moment, and helping them out this way will definitely relieve a lot of stress. So, if you're the ADC, find a moment of safety and tell your support to go help the losing lane a bit. Same thing if you're the support, just tell the ADC you're leaving for a bit to help other lanes, and go help other lanes. If a support is good at roaming, that's a symbol of a damn good support player. If you're mid though, just go by standard proceedure and gank whenever the opportunity arises. Basically, just try to help whatever lane is doing poorly, however you can. Don't sacrifice yourself for them though, especially if you're doing good already.
If it's the jungler that's dying a lot, well then that's a whole other can of peas. Junglers usually die the most to counter junglers. And then you kinda need every lane helping out to solve the problem. You have to follow the standard Anti-Counter Jungle proceedure and ward everywhere. If the enemy is ever heading into your jungle to bother your jungler, you drop what you're doing and go help immediately. Very few people know how to deal with counter jungling, but that's how you do it. Warding, and actively helping your jungler not die. Of course, in certain cases, you don't really want to leave your turret. If 3 people are sieging and the other two are going after your jungler though, you're usually better of defending the jungler than the turret, especially if you're defending alone.
counters 
. Not because Lux can insta-burst Veigar, but because she outranges him. A good Lux keeps her distance, poking him down and executing with Final Spark. A bad one goes close to hit easier, then gets instantly killed.