In Taylor Swift’s world, boys are evil. The 19-year-old country star has sold millions of copies of her two albums, in which she excoriates the mendacious heartbreakers who ruined her young life. So it was a little shocking when John Mayer, who has broken a few actresses’ hearts along the way, made a surprise appearance on stage during her sold-out Staples Center concert in Los Angeles on Friday.  
     
The pair duetted on Mayer’s “Your Body is a Wonderland” and then Swift’s “White Horse.” 
     
“Is there a better place to be in Los Angeles tonight?” the rock ‘n’ roll lothario asked, somehow managing to outdraw Swift in the quest for more ear-splitting shrieks from the girls and young women in the 20,000-capacity basketball arena. 
     
Swift, the top-selling artist in the United States last year thanks to her No. 1 album “Fearless,” kicked off her first headlining tour a month ago. Dates are on tap through Oct. 10 in Minneapolis/St. Paul. 
     
She acknowledged the difficult economic times, and pulled out the stops to give fans value for money. The show ran for just over two hours, and boasted several different sets and at least seven costume changes. At one point, she reappeared at the far end of the venue, playing several tunes close up for fans in the cheap seats, and glad-handing those on the floor as she casually strolled back to the main stage. 
     
After a bombastic home stretch of finger-pointing tunes including “You’re Not Sorry,” “Picture to Burn” and finally “Should’ve Said No,” the show ended on a soggy note. Literally. A waterfall appeared on stage, with the word “No” projected onto the cascade. Swift placed herself directly underneath as she belted out the last lines of her defiant ode to a cheating boyfriend. The message was clear: Her songs are a cleansing ritual, and the screaming women soaked it all up.