"It is useless to discuss with him."
Translation:Cela ne sert à rien de discuter avec lui.
I'm confused. For a previous similar sentence: "it is useless to discuss with her" they did not accept "il est inutile" but they did accept " C'est inutile." But for this one it was just the other way around: "c'est inutile" was wrong and "il est inutile" was correct. So which is actually correct?
Not quite. Grammatically, nothing wrong but "parler à" is different from "discuter avec". A discussion implies and exchange, so you would use "parler avec" rather than "parler à" to mean it. In addition, I am not sure Duo would accept it, because they always prefer more direct translations: discuter/discuss.