It would have been far better if he had got representation before the Court hearing. From what you write it seems as if he doesn't understand the benefit system. Either that, or he was deliberately trying to defraud the system.
Either way, he needs someone to enter a mitigation plea, which the Court will then take into account in deciding his sentence. I would guess that, as he pleaded not guilty but was found guilty, that he is unlikely to be able to make this plea himself as well as it could be made. So, if he wants to be able to minimise his sentence, yes he does need a solicitor. This will cost him (unless he is entitled to legal aid, which is not likely to be granted when the Court has already found him guilty) & - if he gets a solicitor - it is inevitable the first thing will be an application to the Court to defer the sentencing to give the solicitor time to understand the position properly.
Any solicitor he gets should be one experienced in criminal law, & preferably knowledgeable about the benefit system. Your local CAB may be able to give him names.
It is possible for these fraud offences to result in prison - as well as a need to pay the money back. Only a solicitor could give him a realistic assessment of the likely sentence in his case.