Why not "her parents"?
In French, the object is assumed to belong to the subject, in this case il. If it was referring to two different people, that would be clearly marked like this: Selon ses parents à elle, il est gentil.
I also put this, thinking it meant that her parents thought her boyfriend was nice. I guess the authors of this sentence had the thought that it was his parents saying he was nice, not hers...
To describe his personality as gentle or mild, you'd say doux in French. Gentle and gentil are faux amis, that is, they look similar but they don't mean the same thing.
You cannot translate IL EST GENTIL in one sentence as 'he is kind' and in another sentence 'he is nice', and then mark one of the sentences WRONG. They are both correct.
"According to his parents, he is kind" is also correct and one of the accepted answers. Typo?