ESA and pay owed from previous employment

I have a client who is owed some money from previous employment when he was working as a PA to a man receiving Direct Payments for his care budget. The man died in April and Social Services has agreed that my client is due some wages but has not actually got round to paying it yet. My client says that he received advice from another benefits advisor that he was unable to claim ESA (he is now sick) whilst this back pay issue was still ongoing. Can someone tell me if this is correct? If so, could you point me to the regulations regarding this as I cannot find anything? Many thanks. Billy

Thanks Tony, something to mull on

If the earnings he is owed are his “normal” earnings relating to a period of work before his job ceased they do not affect any income related ESA.