Further to above I had a long conversation with AF in Australia.
The bottom line is that if the customer rings BEFORE departure on the first sector to cancel the return they would certainly look at recharging the ticket on a one way basis.
If the customer rings after departure and before return they would say thanks very much and offer a change with 50 euro penalty or complete cancellation with no penalty.
They have no legal recourse and would not want to follow up.
If the customer becomes a no show they just drop it off the system.
Out of courtesy I would notify AF after departure.
The agent said it happens all the time.
She checked with her supervisor that the info was correct and verified it was.
Again thanks to those interested. Regards ML