They went out for a long time, but he found someone else he liked and broke up with her, which didn't seem very fair.
この場合、次のような言い方ができますよ。
ーThey went out for a long time, but he found someone else he liked and broke up with her, which didn't seem very fair.
「彼らは長期間付き合ってたが、彼に好きな人ができて彼女を振るなんて、ずるいよ」
didn't seem very fair を使って「ずるい」を表現できます。
ご参考まで！
カナダ 
