I'm really sorry but none of this makes sense.
"2) He didn't know he had a dissertation supervisor - perhaps because he couldn't access the lectures. "
No. He should know who his dissertation supervisor is. He should submit and proposal and have a set number of meetings to discuss, give advice and review progress. This can all be online by email. Covid wouldn't affect this.
Did he submit his proposal?
A draft? Did he get advice on how to improve his draft?
"He couldn't meet with anyone on his course during lockdown. He submitted his draft and other students did get marked feedback. "
Did he? He must have. If he didn't get any feedback I would have emailed every single tutor I had, the head of dept, the area head, the complaints dept everything.
He submitted a draft, got advice and then submitted his final version.
Is he honestly claiming he didn't know he had a dissertation supervisor. That's ridiculous. Of course he knew.
I'm sorry but this all sounds very weak.