Benefit cap applied following end of 39 week SMP and before returning to work after 52 week Maternity leave- lawful?

My client is a single mother with 4 children, last one born   November 2018. Her UC was capped after her SMP stopped although she was still on ordinary maternity leave and also paid during this ordinary leave,  as the employer owed her leave pay from previous period. Should she not have been treated as still in work and hence benefit cap lifted? DWP have refused to accept MR of decision and not lifted benefit cap until she returned to work one year after maternity leave started. 

Ideas please -

Also, how do you get UC to accept MR? Threaten JR?

It’s about earnings rather than ‘in work’ so as long as she’s earning £569pcm+ (whether ordinary earnings/smp/holiday pay) she’d be exempt from the cap. During unpaid maternity leave she’s employed but not ‘in work’ such as qualifying for childcare costs etc

Does she qualify for the 9m grace period if she earned over £569pcm EVERY month for the last year (£541 before April 2019)