How can one navigate the experience of Tanha I have a no close internet friend who just got stabbed.

Now, seemingly there's nothing more to it than he got in an argument, things were said, and the guy stabbed him. So, my friend doesn't want to involve the police, doesn't want to get any kind of angry revenge toward the perp, he just wants to let it be and carry on doing what he was but with less anger.

Now, for me this seems like an amazing attitude, whether or not he's being hypocritical - which he may not be, it's amazing to have that self control and not even thirst for anything - any kind of sick justice or even self concern.

So, I wondered if there was any way to control my thirsty anger at this turn of events? I don't mean I'm gonna beat the perp up (I don't even know who it was) but yeah, it's wrong to feel anger on someone's behalf, right?

Thanks.
The answer has already been chosen, but I really wanted to provide a personal experience that may help. 

Back in my 20s, I was in a Buddhist temple and witnessed a woman getting stabbed and raped. I was able to help her get away, but the angst from that 5 minute event haunted me for years. I would wake up in sweats and even get into fights with abusive men simply because I was still so angry. 

What I finally did was go into deep meditation and picture the event as a bubble that would float away. I then pictured myself without that bubble. I (mentally) let it go. 

It goes back to what the Buddha first called the Forefold Truths, or the "4" in early language. First, you watch anger, emotions, (suffering) arise but instead of attaching to them, you understand that they are just something that's separate from you. You then use meditation to let them go and when you fully let go, you then have a chance at nirvana or total freedom. Keeping that practice of seeing, understanding and letting go is then the eightfold path. 

In essence, being Buddhist is being at peace.