My nephew’s mom asked if I know why he’s depressed

He’s staying with my for the summer and she asked if I knew why he was depressed. And I do, we’ve talked about it a lot. And idk how to tell her that it’s basically because of her and how she treats him + their living environment. He’s going home soon and it’s really hard to send him back there knowing how unhappy he is. He’s been crying and just generally kind of down as it gets closer.

I don’t know what to say to her. Obviously I would never tell her what he’s said to me in confidence and I don’t want to make things worse for him but idk, I just wish I could help more. I don’t know what to do. He’s a teen, I’m late 20s.