I'm not a lawyer. Sorry this is long, I can't lay it out better.
Strongly advise him to get hold of the RHA (road haulage association) and see if their legal team will take a look at if he's liable here.*
I'm afraid ACAS these days often seem to be more about resolution regardless of circumstances, and will pick up on dodgy method of deduction etc but I'm not sure they will look hard at if he should or shouldn't be paying in the first place.
Plenty of transport lawyers will though.
Sending out another driver to attempt to pull him out as first port of call rather than automatically paying out for a full recovery operation when it very well may not be needed normally, is reasonable enough if it's a simple driver hit mud, soft bank etc. (old school)
Put as simply as I can, given what you've described, for this to be driver/s error in how they attempted a basic rescue, the wheel that caught fire needs to be a drive wheel engaged with the engine. (in which case they've potentially done all sorts wrong from the description)
But a front wheel on most UK trucks/lorries isn't a drive wheel.
Is this a rigid Lorry, or an articulated (tractor) cab towing a trailer?
You said "passenger side wheels" Which ones? If this is an artic, do you know where the fifth wheel is, and can you say if the wheel that caught fire is in front of or behind it, because if its the front wheel it's highly unlikely to be a drive wheel. (which makes what happened not a predictable incident that better care could have prevented, if it's the rear it is a drive wheel, and they've done things wrong, but HTH could that one catch the cab curtains?)
His maximum driving hours are 11, not 12.
His maximum operating hours are 14, not 15.
If he's doing more, he's over tacho rules, and the companies operating license is in trouble if the trafic commissioner took a look. (A much bigger issue than £2000ish to a transport company.)
Either he or the operations manager isn't planning journeys well if he's frequently having to have other drivers sent out because he's out of hours... Not illegal, but raises questions.
*But, there is so much missing from this story.
Drag towed from the front (?) while he was outside beside the truck digging mud away, and a front wheel caught fire? Which happened swiftly enough to have jumped and caught the curtains, and thus set the cab alight? Any insurance company would be looking closely at both cause and odds, because it doesn't quite add up as stands.
So, so many questions here, not least of which is an HGV is likely to be rear wheel drive, which if that assumption is correct, makes it really hard for a front wheel embedded in the mud to be spinning sufficiently to catch fire. So what else was going on, and did what caused him to go into the mud/ditch in the first place have any relevance, etc
I'm not suggesting any intentional untruths, I'm looking for what either driver could have done negligently to predictably cause this set of events, and not seeing it, if this isn't a drive wheel.
A driver could well be liable for any direct damage / recovery costs as a consequence of poor driving putting the truck in the mud/ditch, if his contract is worded to cover that.
But, can he be held liable for recovery damage caused by one driver helping another out at the bosses behest? What's their contracts say? There are levels of knowledge and common sense required when carrying out such actions, and if the wheel that caught fire was a drive wheel, then there's all sorts they should have known, and the situation could have been predictable to some level, but if it isn't a drive wheel, nothing they've done as described could imo be predicted to lead to a fire.