It was one of his so-called friends who supplied him with the drugs that killed him.

so-called 是由 so（如此）和 called（被称作的）组成的复合形容词，字面意思是"被如此称呼的"。 so-called 通常带有讽刺或怀疑的语气，表示"所谓的"，暗示实际可能并非如此或名不副实。