In my view the websites giving info targeted for employers are just as good for employees - 'cos they explain the process, what the steps ought to be, and what should be documented etc. The same applies to both sides.
If you want to talk specifics, post again.
If he has been issued with disciplinary action (a Warning of some type, perhaps?) the employer should have first undertaken an investigation to consider the situation, and have interviewed him to get his side of the story. That really was his only opportunity to explain the circumstances in his defence.
If he now got some Warning issued, and if he really isn't happy about either the process they followed or the conclusion/decision reached, then he should appeal. That too is part of the ACAS Code of Conduct for Disciplinary Action..