Tumeric in curry is used as a dye and will set with heat, so do not hot water extract,
Funnily enough its not a very good dye and fades in light, I have known carpet cleaners in the past place an ultra violet light over the stain and its been removed to a hefty degree ,I have not done this myself but I can see the logic.
If the customer has attempted removal, he's blown it ,you will not remove it and I would emphasise the point before you start that you are only attempting a removal at his request but the chances are slim. I would not waste too much effort or solutions on it just make a token attempt at removal on a small section of the stain.
Its probably an insurance claim anyway and he will want a new carpet.