Accept the letter apparently states:
That appears to be stating fact, 'illegal websites' (plural and were they illegal?), 'accessing' (asserting something did happen). If you weren't sure of this being the case and needed to investigate further I'm not sure you'd state it as though it were fact. If he was suspended on full pay for suspected breach of contract pending further investigation wouldn't he have his disciplinary hearing post investigation? It sounded as though he's had a disciplinary that he took a colleague in with him for but maybe this was just for his suspension? Whether it constitutes 'Gross Misconduct' is also somewhat of a gray area as I think the employer would need to demonstrate how said action caused a tangible loss to the business.
Standard procedure is that after an incident has been reported HR will hold an initial meeting to discuss the allegations and suspend the employee while it's investigated. You'll get a letter explaining it. It's rare you'll take or get to take anyone in here as it's just a meeting to inform you what is happening.
You'll then be invited to a second interview where evidence will be presented, your statement can be recorded. It's normal to take someone into these meetings, ideally a union rep, but they can't answer questions on your behalf but can but in if the meeting isn't being conducted properly. They should provide you with evidence collected before you attend so that you can build a defense.
Once this is done, you'll either be awaiting a second meeting for further evidence or just the outcome of the meeting.
As for the OP's relative, it appears that he's had the initial meeting informing him of the investigation.
The issue here is, what system did the person log on to? How are these sites illegal? Unless it's some creepy porn then regular porn or torrent sites are not illegal, even if they go against company policy.
The only illegal thing that appears to have happened in this case is that someone has read his personal emails, which, even the companies IT department are not allowed to do.