This is book #2 in the award-winning Faith & Kung Fu series by T.M. Gaouette
 
With Tanner Rose gone, Gabriel is left to settle back into the God-devoted life he’d become so accustomed to. Except that his best friend’s sister, Faith, has decided that a life for the Lord is just way too confining. In an attempt to live a little, Faith focuses her interests on a handsome, but worldly, boy from her school, who has major self-control issues in and out of the kung fu arena. When things get too deep for Faith, she looks to Gabriel for help. Can Gabriel save Faith from self-created destruction or is he just too lost in his own worldly struggles to lend a hand?
"Do not be conformed to this world, but be transformed by the renewing of your minds, so that you may discern what is the will of God—what is good and acceptable and perfect." (Romans 12:2)