Journalists commonly say that readers typically don't read past the first paragraph. Apparently so. Let me quote paragraph 2 of the article in total:
Julie Lake, 50, believed the 15-year-old was one of a number of youths who had damaged the remembrance garden in her village dedicated to those killed fighting for Britain.
The woman confronted the kid who she thought was one of the kids involved in a series of past crimes. No ongoing crime, just him, and her, and her temper.
If a person intervenes to prevent an ongoing crime, that's legal, and commendable. If the same person, after the pursesnatching or whatever has been stopped, turns around and slugs the perpetrator because of a fit of pique, that's battery. If a person recognizes (or, as in this case, thinks he recognizes) someone who he saw snatching a purse a while ago and physically attacks him, that's assault. And maybe battery too.
When outrage crosses the line to attack without legal authority, the word we use is "vigilantism". Now, if we want to live in a world without law...