HMRC and employer posting earnings

Hello all,
I was wondering if anyone has had a similar case or any help please.

We had a tenant who worked for an independent fast food outlet. The outlet closed down in August and the tenant moved onto UC. The tenant informed us that she had real issues with payments from the outlet and only got paid fully once when she started and her final months wage was only £50.

Her 1st UC payment was very low due to the HMRC posting that she had earned £600+ for August 31st and again for September 31st which will affect her next UC payment.

Unfortunately she has no P45 and only ever received 1 payslip the rest of the money was in cash.
To be fair to the UC centre they are looking into it as much as they can but wanted to know if there is anything else that could be done to help.

The only thing I can suggest is ask UC to send a breakdown of the £600 which I am told is a screen shot from the HMRC system.

But if your clients account differs they are going to struggle to prove without bank statement or payslips.  Written statement is worth a try. Or going back to the employer to ask them to correct?