Most importantly he's got to be willing to mute people if they start talking shit. It's not worth engaging with at all. And with voice chat now implemented, you can even type /mute all and rely on voice with your premade to communicate.
On actually dealing with toxicity though, for someone who wants to do something about it or attempt to stop / redirect it. The biggest thing about flaming is that it's all done in the past tense. Someone who's flaming is almost exclusively talking about something that already happened, that cannot be changed, and that cannot be fixed. Sometimes they will use the future tense, but simply be referring back to what happened in a flaming manner. Simply put, talking about things that already happened is like wasting words that could instead be used to come up with strategies to win in the future.
If you can refocus the conversation on what to do in the future then you can stop a flame war in its tracks. Start by saying that talking about what already happened is useless, "it's already done, no changing it now." Then suggest or query others for a strategy that can be used to succeed in the future. For example, "lets see what we can do to prevent that next time, how about we ward the paths so we can see if they are coming to attack?" Or, "if you play safe and wait for farm under turret you will be able to get more of it without being attacked as much" and last example, "If you can hang tight for a few more minutes we will roam and help relieve the pressure you're facing."
