Vigilantes aren't patted on the back and sent home, he'd need to be questioned extensively, and likely would be handled as a suspect until they get the story straight. I doubt they'd go so far as to keep him in a holding cell or book him until it was justified, but walking up and seeing a guy holding some one at knife point...I don't care what the other guy looks like, both of them are getting cuffed.
"Good thing some people carry knives!"
...This is a cop saying this? They're encouraging knife use of this manner?
...Warrants for six rapes and two murders? This could just be the wrong wording. If he was charged after being caught, that would mean he'd later be identified as a serial attacker. Serials are never just scooped off the street, you're processing them for a single attack, when it all comes falling out via confession or further investigations. Warrants, are directed at a person with a name and a face. If he had eight life-threatening warrants out on him, he'd practically be on 'America's Most Wanted, he'd be a fugitive who'd be best out of the country'. Your friend would be in for a year or two of testifying in court as they come down on a guy for a life sentence or death penalty if this happened.
The details about running with a multi-tool also seem odd. Even us blade-philes keep it light if we're moving around, and the Wave/Charge line is quite large. Finally...the fact your friend is apparently a ninja who snuck up to within inches of a man while he was obviously moving around quite a bit. The way the guy just stood there obediently for quite a bit of time is also odd, if he's experienced he'd at least make a grab for it.