Just try a small patch (maybe the size of your palm) choose the worse area. If you get a good result after solvent stain removal and rinse, then do the rest and charge.
Doing a small patch is only going take 10 minutes (so no need to charge if it was me) to know if its not happening and just explain this to the client. And make sure you explain well!! before and after attempting a clean!!
I did a clean recently for an asian couple that claimed that i created a stain that wasn't there!
Bedroom carpet was grubby all over no visual signs of any particular stain, this wasnt helped by the fact that it was coated in rug doctor shampoo from a previous clean attempt! Once i cleaned the carpet beautifly this blooming green stain appeared on the carpet from no where!
p.s. This will probably be looked down by some of you guys and the likes of the NCCA but i have caked and i mean caked stains with solvents (mainly gels) with long dwell times i may add! on all types of carpet 100% wool, 80/20's, polys, berber etc over the years and never had any problems or come backs.
There's alot of scaremainering going about.
Tony