It's such a pity that he can't realize his potential because of a bad relationship between he and his teacher!
ご質問ありがとうございます。
"It's such a pity"＝「それは本当に残念だ」
"that he can't realize his potential"＝「彼が（自分の）力を発揮できないとは」
"because of a bad relationship between he and his teacher!"＝「彼の先生との悪い関係によって。」
（又は、"because of human relationships!"）
ご参考に。
アメリカ合衆国 
