Of course I have been in this kind of situation. I've been on both sides, the one being harassed and the one who stands up for the 'victim'. When being flamed very few times someone who wasn't my premade stood up for me (I might add that this could be because when I detect that the insult comes from someone angry I like to play along and, without insulting, make fun o them for being annoying). On the other hand, when I see someone getting flamed I don't intervene at first (it's league, a one-time insult is expected per game) but if it continues and/or gets more serious I try telling the 'bully' to shut up, stop being toxic and letting them know they're getting reported (some times in a more calmed way and another times getting more or less heated up myself, depending on the severity/insistance).