There is a lady at work who always thinks people are a couple whenever she sees two single people together.
この場合、次のような言い方ができると思います。
ーThere is a lady at work who always thinks people are a couple whenever she sees two single people together.
「職場に独身の人が一緒にいると付き合ってるとすぐ思う人がいる」
to think people are a couple で「人がカップルだと思う・付き合っていると思う」
single people で「独身の人たち」
ご参考まで!