They will note take your relative's belongings, don't worry - they have no powers to do that as they are not bailiffs.
You will need to make a complaint to the energy company and tell the debt collectors you are doing this too. Ask the debt collection agency to pause any enforcement action pending the outcome of the complaint.
You should go on to the supplier's website to find the complaints procedure and start that as soon as possible - and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response.
Once you have the deadlock letter the next step would be to escalate this to the Energy Ombudsman - they will investigate and liaise with the energy company which would hopefully result in a cancellation with no bill to pay. They can order the energy company to make a financial award for inconvenience if they have acted poorly.
The service is covered by the Consumer Rights Act 2015 so you have a right to expect a reasonable service which is carried out with care and skill - if you do not then the Ombudsman will step in to help you with this.
Once you have the final response from the supplier you can complain to the Energy Ombudsman by email here: ***@******.***
Or via their website here: www.ombudsman-services.org/energy-complaint.html
If the debt collection agency continues to contact you, just note that debt collectors are not bailiffs - they have no powers as such. They will simply try to collect the money their client thinks you owe. You can turn them away if you dispute the debt and they visit you - they can’t enter your home and take goods. They would report back to their client they were unsuccessful and it’s then up to their client whether to take matters further.
If the supplier decides to sue (assuming the Ombudsman does not uphold your complaint), you will receive court documents (a response pack) which you must complete and return to the court. It is a tick box exercise for the most part and there is a short section to write a defence which is easy enough though please feel free to come back to this site if you need any more help. A claim will also take 9-12 months to be decided at court. If you lost then you would get 14 days to pay the judgment before the claimant can enforce the order, and 30 days to pay in full before it is registered with credit agencies. The claimant cannot recover legal costs if they win, in a claim under £10K (a small claim), all they can claim are the court fees and interest.
If the claim has no merit then you have an option (after your defence is filed) to apply to strike the claim out. The court will consider an application if the claim has no merit, or is misconceived. The application costs £255 but this is recoverable if your application succeeds. If you are on a low income, have low savings or in receipt of benefits then you can ask the court to waive the court fee. If you won the application, the claim is struck out.
The hearing (if the case gets that far) is likely to be held remotely, it's you, a district judge (who is a practising solicitor or barrister) and someone from the claimant company. The Judge decides and if you lost, you get 14 days to pay the sum. If paid in full within 30 days then nothing goes on your credit record. But certainly I think you would succeed in a defence, just to be clear.
If you lost and if you were unable to pay the full sum owed at that point you can also ask the court to pay by instalments (a simple form is sent in, form N245, and a fee of £50 unless you qualify for a fee exemption so if you are on a low income, have low savings or in receipt of benefits then you would qualify). If the CCJ was not paid in full though bear in mind the credit record would contain the CCJ details for up to 6 years. After that it comes off the credit registers.
I can assist you going forwards if it gets to the point they issue a claim. If you need a law firm to assist you to defend the claim, here is an example for you and they offer a fixed fee : https://stormcatcher.co.uk/practice-areas/dispute-resolution/small-claims-lawyer-service/