Hello Homer,
The sponginess you refer to is also called delamination. It's not uncommon in caravans especially in high traffic areas like the door way and the kitchen areas.
There are repair kits for DIY and professional to use, and many on this forum have reported good results with them.
But the issue you bring relates to the responsibility of the caravan manufacture in these cases. Legally you only have redress against the person or business that sold you the caravan, i.e the seller. And you do have rights under 2015 the Consumer Rights Act.(CRA)
Essentially if a seller supplies goods that fail unreasonably within six years of the date of purchase, the customer has some rights to expect the seller to put it right. There are a number of steps you need to go though to satisfy the terms of the act, but in this case, the crucial point about whether the design or workmanship was adequately durable has been answered by the fact the repair agent has acknowledged the repair needs to be done, and they have refereed it to the manufacturer.I assume under the Manufacturers Guarantee. Did they offer you a choice to use the Manufactures Guarantee or your statutory rights under the CRA? Ill bet not!.
Now you have alluded the fact the caravan was purchased in 2016 but you don't tell us exactly when, becasue if you reported the issue within 6 months of from the date of accepting the caravan, your brother is in a very strong position to demand the seller repairs the caravan under the CRA - not the manufacturer guarantee.
Under the CRA faults that appear within 6 months are deemed to have been present or inherent at the time of taking control of the goods.such the seller must initiate repairs as soon as possible, and it does not matter what the manufacture thinks or says about it, it is the sellers responsibility to do it with the minimum of inconvenience or expense to the customer. That include collection and delivery to and from the dealers for assessment and repairs.
If its over six months, then the customer must be able to show the design was faulty or the workmanship was poor, or faulty materials were used, but as a repair shop has already identified the product has failed, that should not be a stumbling block.
Do not feel sad for the seller. The law is clear, they should not sell faulty goods. They have done, and they have made a profit on your brother's caravan, even though it is faulty. That is a breach of the CRA.
They had every opportunity to fully inspect the caravan for faults before it was sold to your brother. If they had done so, the fault would have been found and the caravan should have been returned to the manufacture for repairs or exchange before it reached your brother.
It is undeniable the manufacturer is the cause of the problem, and morally they should be held to account, but liability lies along the route of the contract, in which case its your brothers contract with the seller.
There is a twist to this, its quite possible your brother may have used a finance package to purchase the caravan, Under section 75 of the Consumer Credit Act , the finance house carries a joint responsibility the goods purchased. So contact them also for support in the matter.