The girl was looking at him in envy, (because he seemed to have everything that she doesn't have.)
The girl looked envious of the boy.
ご質問ありがとうございます。
①
"The girl was looking at him in envy."=「少女は彼をうらやましそうに見ていた。」
"because he seemed to have everything"=「なぜかというと、彼は全て持っているようだった」
"that she does not have."=「彼女が持っていないものを」
②
"The girl looked envious of the boy."=「少女は男の子に対して羨ましそうだった。」