If Swift have declined then this presumably shows they have no records of annual service. The Dealer who sold it originally is they only one with any obligation for bodywork up to 6 years old, if it has full service record. If they have serviced it every year from new then you may have a chance of them fixing it. Otherwise may have to get it fixed privately at cost.
This would be the case if the the present owner had purchased the caravan from the dealer, Under the Consumer Rights Act, (CRA) the Dealers' obligation is to the retail contract they shared with the original new purchaser. If purchaser sells the goods privately, the CRA contract expires, and the dealer has no contractual obligation to the most recent purchaser.
This is entirely separate to any obligations created by the Manufacturer's Warranty's or guarantees. As the name implies its a contract between the manufacture (Not the dealer) and the original purchaser. What is often misunderstood, is that there is no legal requirement for the manufacturer to offer a manufacturers warranty, as such there are no statutory regulations that cover these warrantee's, which allows the manufacturer to control the limitations of these policies. Most UK caravan manufacturers have chosen to offer these warranties, but they are are usually contingent on the caravan owner having the caravan "serviced" and "Inspected" by "approved workshops" in accordance with a set time table based on the date of original ownership. Failure to comply can result in the manufacturer's warranty being declared null and void.
Fortunately most caravan manufacturers will allow any unexpired manufacturer's warranty to be passed on to a new owner when a caravan is sold - even privately, but it is usually contingent of there being a full uninterrupted record of approved service and inspections. How that evidence is collected, has and is changing. Electronic records ae becoming more common. Approved workshops should have the means to update the records online.
With a broken record of service and inspections', its very unlikely the manufacturer would entertain any claim for manufacturers warranty work whether it's from the original or a second user.
The only other route I can think of is to consider if your friend wanted to take a small claims court action against the private seller, on the basis of selling faulty goods - but that would be a difficult course to prove. You would need to be able to show the seller was aware of the defect and deliberately chose to hide the knowledge or evidence. I this route is chose it would be best to seek professional legal advice.
I'm sorry AnnP but suspect your friend is on their own on this instance.
Your friend should contact an member of the
Approved WorkShop Scheme to find a qualified repairer who can quote or advise about how to get the caravan repaired.