I've just come across this story and here are my thoughts:
Please note, this is my belief and not legal advice.
Did you purchase the entire setup via the installer?
Is it a genuine company or a mate trying his/her best?
Did you pay for any or all of the job with a credit card?
If a company supplied and installed the lot, they have a duty of care towards you. Talk to your trading standards people. They'll need dates, contact details etc so hopefully you kept everything like receipts, appointment letters/emails etc. See this government website page
https://www.gov.uk/consumer-protection-rights
If it was a mate then you could be on a hiding to nothing. It's unrealistic to expect a manufacturer to replace items they made, perfectly well, that have been damaged or abused by someone else. Imagine you sold a car to someone. Some time later they come asking for their money back because they've crashed it and it doesn't drive now. It's not going to happen. You could try a small claims court action against your mate but you won't be mates for too long afterwards.
If it was a non-responsive company that supplied/installed the system AND you payed for part/all of the work by credit card, contact your credit card company. In effect I believe the law says that the credit card company bought/paid for the work and you then bought the job from them. They are at least in part liable for reimbursing you. Tell the card company the dates and times you tried to contact the installer and any responses. They'll expect you to have tried your best to get the job put right but if you're up against a brick wall, they'll normally take over and you'll be reimbursed.
Note that buying on finance (bank loan, HP etc) is not the same as using a credit card, even if the company doing the job arranged finance for you from a third party. Whereas using a credit card is as above (the card company bought the job then sold it on to you), using finance means someone loaned you the money for you to directly buy the job. You can't withhold payment to a finance company or bank. They had no input into the job but simply loaned you the money to get something done. They can't be held responsible for something going wrong with the job and you must repay them.
You mustn't attempt repair of any of the job. If you do then your installer could argue that he/she did the job properly and you have upset the apple cart. Like crashing the car (above) you can't expect him to carry out remedial work and it'll be your word against his that he did something wrong.
Finally, what do you believe he did wrong? From what I see, it would be easy to reverse polarise the batteries which could be extremely dangerous as they'd possibly go into thermal runaway (that would be making the mistake of black to red, red to black, plug it in and stand well back) but they can't be parallel connected and connected to the BMS at the same time. If two batteries were inadvertently parallel connected then as long as they were both in a similar state of charge at the time there ought to be little to worry about. If on the other hand, one was fully charged and the other very low it could cause a massive current outrush from the charged and an inrush to the uncharged, possibly taking out any protection fuses. One must be vary careful how one charges litium batteries. I recall a time , years ago, when a tecchie inadvertently reverse charged a lithium battery resulting in an explosion that blew out the window in the room.