How can someone seek help to get out of a unit?

Based upon what was written have your friend or yourself seek mental health treatment. When everyone is messed up, but the individual that is a huge red flag. You said they already see mental health. So the question becomes what makes you think you are more qualified than the provider? Is it because your friend is completely honest or untruthful to them, but somehow you have all the info and no one is misinforming you. That isn't logical as you are subject to a lot of misinformation yourself. You are either being suckered by your friend or assuming you know a lot more than you do. If you want to help the person recognize that they are deeply troubled and not everything they say can be taken at face value. They need help and you are not a mental health professional, but can be a friend. So you cannot be everything to everyone and you also cannot assume that everyone involved is an idiot with bad intentions, but somehow you are seeing what everyone else is missing.

If your buddy claims to have witnessed a sexual assault, chances are high that he is actually the victim. He can be transferred very rapidly as a SHARP victim and all he has to do is a restricted report and request a transfer.

If he's not willing to come forward to Chain of Command or IG then you could speak with the IG as a third party. Ultimately their hands will be tied if James doesn't want to cooperate but I'll buy you lunch if this is an isolated incident and/or there is not a lot of cover up that an investigation won't reveal. James may be hiding, but this is tough stuff to cover up and hide behind. Good luck.

There is nothing you can do by helping him "hide' all that has happened to him.. Without a so called victim you do not have a real issue. "James" needs to be the one to decide to deal with the issues if he wants to, if not, then it is his problem to shoulder and not yours.