Expediente literally means 'record' or 'file', often referring to an official document or case file. It's an interesting word choice in a song about a relationship.
In the lyrics, the singer says, "Si yo sabía tu expediente" (If I knew your record). This implies that he was aware of his partner's past behaviors or reputation, suggesting she had a history of being problematic in relationships. It's a clever and somewhat formal way to express knowing someone's 'track record' in a romantic context.
Luister La Voz spends most of this song sounding relieved rather than sad. The hook is one short phrase repeated like someone talking themselves down from a bad week: "Menos mal / Que Dios me sacó de ahí" (Thank goodness God got me out of there). He opens by admitting the part that still stings, "Me arrepiento de haber sido vulnerable" (I regret having been vulnerable), and then spends the rest of the verse deciding the problem was hers, because "Le quedó grande saber amar" (knowing how to love was too much for her).
Manuel Turizo's verse is where it gets petty in a satisfying way. He tells her to keep the watch, "Quédate con el Rolex y el tiempo que me quitaste" (Keep the Rolex and the time you took from me), counts up the holiday photos she posted, and lands on the line the whole song has been walking toward: "Solo eres buena cuando mientes" (You're only good when you lie). The chorus answers it bluntly, "Si no da todo, mejor no des nada" (If you don't give everything, better give nothing), which is a lot calmer than most breakup songs manage.