Reciprocal agreements and UC

Anyone have thoughts around UC and reciprocal agreements? CPAG Migrant Handbook suggests that it exists, but the person needs to have a ‘right to reside’ to claim the benefit.

Client in particular has dual turkish and eu nationality. DWP are so far not buying that she is a worker - does her Turkish nationality provide an additional avenue? Her turkish passport does not have any visa for the UK in it as she’s always been in the as an EU national.

She has two british children, one dependent so no derivative options and permanent residence is a bit shaky. 

Thanks very much.