The coach may decide to involve other authorities if he becomes more certain about the abuse. He could talk to the school counselor or even the principal. They can then work together to ensure Stiles' safety and get to the bottom of what's really going on.
Stiles' withdrawn behavior could also be a sign. If he used to be outgoing and suddenly becomes very quiet, avoids social interactions, and seems jumpy or nervous all the time. This change in behavior can be an indication that something bad is happening to him at home or elsewhere, which might lead the coach to think he's being abused.
He could overcome it through the support of his friends. If his friends are there for him, they can offer a safe space and positive reinforcement. For instance, they can remind him that he is not to blame for the abuse.
It could be that Stiles is in some kind of danger, perhaps from another supernatural being. And he goes to Peter because Peter has certain powers or knowledge that could protect him. For example, Peter might know how to defeat a particular enemy or how to get out of a tricky situation involving magic or some other supernatural element.
It's not okay. 'Stiles' is a character that fans love, and writing about him being abused in fanfiction can have a negative impact on the overall perception of the character. Moreover, it can trigger uncomfortable feelings in the readers, especially those who are sensitive to such topics.
One way is for the pack members to stand up for Stiles more often. If someone outside the pack is giving him a hard time, they should be there to protect him. Also, they could share their secrets with him, which would show that they trust him. This trust is an important part of any relationship and would help Stiles realize that they don't actually hate him.
One way is to have clear guidelines in the fan community. The community can set rules that forbid the creation of such disturbing content. For example, fanfiction platforms can have strict content policies against depicting abuse.
On the other hand, Stiles is also known for his quick thinking. He might start to try and bargain with Peter. He could offer to help Peter in some way if Peter lets him go. For example, if there's a problem in the supernatural community that Peter is facing, Stiles could say he'll use his human ingenuity to solve it. Also, Stiles could be angry. He might start shouting at Peter, calling him out on his actions, and trying to make Peter see how wrong this is.
Stiles usually reacts with a mix of fear and defiance. He's afraid because he's in a dangerous situation, but his strong - willed nature makes him defiant. For example, in many fanfictions, he'll shout at Peter, trying to show that he won't be easily broken.
He could use his intelligence to manipulate Kate. Maybe he convinces her that he is more useful to her if he is free, and she lets her guard down long enough for him to make a run for it. Also, he could have some hidden skills or knowledge that he uses to break free. Say he knows a spell or a code that unlocks his restraints or the door to his prison.
There could have been some misunderstandings. Perhaps Stiles overheard a conversation that he misinterpreted. Or some actions of the pack members were misconstrued by Stiles. For instance, if they were joking around but Stiles took it the wrong way and thought they were being mean on purpose, which made him think they hated him.