I first met her at Hirai Ken's concert and that's how we first became friends.
I became friends with her after first meeting her at Hirai Ken’s concert.
We first met at Hirai Ken's concert and we became good friends after that chance meeting.
「友達とは平井堅のコンサートをきっかけに出会った。」＝ I first met her at Hirai Ken's concert and that's how we first became friends. / I became friends with her after first meeting her at Hirai Ken’s concert. / We first met at Hirai Ken's concert and we became good friends after that chance meeting.
ボキャブラリー
first met / first meeting ＝ 出会った
became friends ＝ 友達になった
chance meeting ＝ 偶然の出会い
イギリス 
