In "心陪着你, 因为我想知道 她(他)是不是比我好"
He used '你'(you) to refer his ex-girlfriend at the beginning of the sentence. It is wrong to use a different pronoun "她" to refer his ex-girlfriend in the same sentence later.
她 has to be a typo, It should be '他' not ''她 in this line
"My heart is with you because I want to know if he is or isn't better than me"
(Implying: If he is, I am happy for you, if he isn't, then come back)
Ps. It is official lyric so I don't think it's a typo
Official lyric can use the wrong word too.
'Made a typo' means the guy who typed the lyric mistakenly typed 她 instead of 他,
'Chosen the wrong word' means the guy who typed the lyric mistakenly thought 'XX' in 'XX 是不是比我好' was referring to the girlfriend instead of the girlfriend's new boyfriend