A friend of his who is a high school student volunteers every day.
A friend of mine who is a female high school student does volunteer work every day.
ーA friend of his who is a high school student volunteers every day.
「彼の友達の高校生は、毎日ボランティアをしている」
a friend of his who is a high school student で「彼の友達の高校生」
to volunteer で「ボランティア活動をする」
ーA friend of mine who is a female high school student does volunteer work every day.
「私の友達の女子高生は、毎日ボランティアをしている」
a friend of mine who is a female high school student で「私の友達の女子高生」
to do volunteer work で「ボランティア活動をする」
ご参考まで!