That lyric is not really special, everyone can say that
You'll find a bunch of similar lyrics in this world
I searched "it's always a good time", and I found Owl City + Carly Rae Jepsen's song, and Travis Emory ( I don't know him )'s song
I doubt it was intentional. She probably heard the song and later thought she came up with the lyric independently. She has shouted him out in a few interviews and wrote his lyrics on her arm during the Speak Now tour so I'm surprised he publicly called her out. Plagiarism is wrong but this not a special line in a song that is full of great lines.
Plagerism is never okay. But can't believe he called her a thief, how is it coincidence since she knows him and probably heard his song. Anyway he can't sue her.