Has he got a contract and any company handbook etc...? If so, what does it say about disciplinary matters, offences, sanctions, procedure etc...
Any procedure and action should be fair and reasonable in the circumstances.
ACAS would be a good place to start for advice and they have guides such as a Code for disciplinary and grievances. They should be able to advise what rights he has, what should be done and how so he can raise issue if it isn't done properly They have a website with lots of information.
Make sure he keeps a written log of everything, dates, times, what was said by whom etc...
If he has a disciplinary hearing, he should be able to bring someone with him, well worth considering and planning ahead as to an appropriate person to take.