Question: Shouldn't "she tells them that she loves him" also be accepted? I have read somewhere that instead of the indirect pronoun "loro", "gli" is often used in modern italian. Therefore, I've read that "gli" means both "to him" / "to them".
Is that right please?
That is correct, but because it can mean both in this situation, it's more likely that she is telling a man that she loves him. It should definitely be accepted though.
She tells him that she loves him. Marked wrong. There are tons of little mistakes, I'm getting tired of reporting
Why are the "him"s different? Is it a matter of "she tells (to) him that she loves him [object]"?
Hmmm, why isn't correct when I say "she says him that she loves it" why we have to use "she says to him..."
Actually, "says to him" is correct and good English. Nothing wrong there. I think Duo was expecting "that she loves him" not "loves it", although that's not wrong either. Given Duo's utter failure to provide context for learning, using "it" instead of "him" is correct also..
I'd report the sentence as you put it. It's perfectly good English.