Hi Terry
This sounds like it could be quite a problematic stain to deal with, and the limitations should be discussed with the client fully prior to proceeding.
Coffee or tea for that matter are always far more problematic when spilt hot, as apposed to being spilt cold. What happens is that the dye sites within the fibres expand and open up, readily accepting the dye or tannin contained within the hot beverage. Upon cooling the dye sites close up encapulating the tannin dye, and mix with the original dye of the carpet. In effect the carpet fibres are no longer their original colour, and no amount of cleaning can restore the changed formation of the carpet dye, mixed with tannin stain.
The clients use of stain remover products and age of the stain, I would suggest are of little consequence, as the heat of the beverage would be the main cause of setting the stain into the fibres in this instance.
The first call would be to rinse the stain with an acetic acid, and the remainder would probably require chemical removal procedures using Oxidising or possibly reducing bleaching agents.
When using these sorts of bleaches you are in the realms of high risk in discolouring or taking colour out of the surrounding area of carpet to the stain, especially when treating coloured carpet fibres.
White, or off white fibres can be treated with less risk of the bleaching effect being visual apparent, but a lightening effect is still very probable, and again the client should be informed of the potential risks, preferably in writing prior to proceding.
Unfortunately, the only time we as professionals can guarantee stain removal is when you take along your Stanley knife!!!
If the stain contained sugar and milk then this may require separate removal procedure for the starch and fats.
I hope that helps in giving a good grounding in understanding what we are up against when dealing with hot dye stains.
Regards
Robert Olifent