清麿 is the name of his friend.
が is used to emphasise the preceding word as well as indicating the subject.
変わった is the ta-form or casual past tense of a verb 変わる meaning “to change”, so 清麿が変わった means “Kiyomaro changed”.
んじゃない is the broken version of のではない. It consists of the nominaliser の and ではない which can be simply translated as “not“. It’s usually used to negate the preceding word or clause and express the speaker is trying to explain the reason or background of the context. So he is trying to explain the reason that Kiyomaro doesn’t have friends anymore is not because he has changed.
A negative sentence with んじゃない is often followed by a correction which, in this case, is 清麿を見る友達の目が変わったんだ.
清麿 is again, the name of his friend, as Japanese tends not to use pronouns like he or she as much as English. The particle を indicates the direct object of the following action which is 見る meaning “to look“ or “see”. So 清麿を見る means “to look at Kiyomaro” and it modifies the following word, 友達の目.
友達 means “friends”, the particle の indicates the possession and 目 means “eyes”, so 清麿を見る友達の目 means “his friends’ eyes looking at Kiyomaro”, which is the subject emphasised by the particle が.
変わった means “changed” and んだ is the opposite to んじゃない so it’s used to affirm the preceding word or clause and express the speaker is trying to explain the reason or background. So 清麿を見る友達の目が変わったんだ directly means “It’s because his friends’ eyes looking at Kiyomaro have changed”, which can be translated as “It’s the way his friends see him that has changed”.
が is used in both sentences as he is emphasising who is the one that has actually changed and んじゃない and んだ are used to express that he is trying to explain the reason or background of the context.