I am sorry to hear about your loss.

I suspect that EK did refund the value of the return segment and likely waived administrative fees. It is very common for a return ticket to cost very little more than the single (and sometimes even less). Thus, the GBP 580 would be the difference between the single she did fly and the cost of her return ticket.

The only way to be sure of this is to call the travel agent your mother used and even EK and ask for a breakdown of the GBP 580. If there is a different between EK's breakdown and the TA's breakdown, then the TA is charging you its own fee to process this.

If indeed you are being charged any fees beyond minimal ones which may not make this effort worth it, those are likely claimable from her travel insurance.