His shirt is too open./This person's shirt is too open.
He unbuttoned his shirt too much./This person unbuttoned his shirt too much.
「この人ワイシャツのボタン開けすぎじゃない?」は英語に訳すとこのようになります。
His shirt is too open./This person's shirt is too open.
He unbuttoned his shirt too much./This person unbuttoned his shirt too much.
「ワイシャツ」は英語では collared shirt に相当しますが、こういう場合には collared shirt を言わなくても大丈夫です。Shirts だけと言った方が良いです。
to unbutton = ボタンを外す
例文
Why did you unbutton your shirt? You should button it up!
なぜシャツのボタンを外したの? ボタンを締めたようが良いですよ。
ご参考までに。