The only free seats were on either side of that person.
The only seats that were empty were on either side of that person.
「(地下鉄の席で)あの人両側の席だけ開いていた。」＝ The only free seats were on either side of that person. / The only seats that were empty were on either side of that person.
ボキャブラリー
only ＝ だけ
free seats/seats that were empty ＝ 空いていた席
either side ＝ 両側
that person ＝ あの人
He was the only person with empty seats on either side of them.
この場合、次のような言い方ができますよ。
ーHe was the only person with empty seats on either side of them.
「両隣が空いていたのは彼だけだった」
empty seats on either side of them で両隣が空席だと言えます。
ご参考まで！
イギリス 
カナダ 
