One possible cause could be the stress from all his past quests. He's been through so much, faced many dangerous situations, and lost some friends along the way.
The seven in this fanfiction are probably a set of characters who are central to the plot along with Percy. They could be his friends or allies in the world of demigods. Maybe they are characters like Annabeth, Jason, Piper, Leo, Hazel, Frank, and others who are on the same side as Percy in the fight against the forces of evil in their fictional world.
The others in the seven, like Jason, would probably feel a sense of guilt for not noticing earlier. They would come together as a group to support Percy. Jason might offer his friendship and strength to Percy, telling him he doesn't have to face it alone anymore. They would also likely start to investigate who was responsible for the abuse.
They would likely be shocked and extremely worried. They might immediately try to stop Percy from doing it again and get him some help, like finding a healer or someone who can offer mental support.
Well, one possible plot development could be that the Seven initially don't believe it. They might think Percy is joking or just going through a rough patch. But as they start to notice his withdrawn behavior, his lack of care for his own safety during quests, they realize the seriousness. Then they would probably try to surround him with love and support, and maybe even seek help from the gods, like asking Aphrodite to restore his love for life or Apollo to heal his mental state.
Piper would use her charmspeak to try and soothe Percy's troubled mind. She'd also be very empathetic, listening to him without judgment. The rest of the Seven would also come together as a group. They'd create a safe space for Percy, where he could express his emotions freely, and they'd all work together to help him see that he is valued and loved.
They might be shocked at first. They could feel a sense of guilt for not noticing earlier. Maybe they immediately try to comfort Percy and offer their support, like giving him a safe place to stay and assuring him that he's not alone anymore.
In this kind of fanfiction, the seven might react with shock. Percy has a complex past filled with battles and losses. They could become more empathetic towards him, or it might cause some friction as they re - evaluate their relationships with Percy.