You can "apply a flame retardant treatment".
You cannot "stop it catching fire" !!
As long as customers understand the difference, fine. A flame retardant (correctly applied) will cause combustible materials to smoulder rather than rapidly burst into flames. This helps to stop fires rapidly spreading and getting out of control. The effect of the product is different according to the material it's applied to. In some cases you'll have trouble actually lighting the fabric at all, in others it just burns less severely.
If someone just wants to avoid getting f@g burns in their sofa cushions, tell them to smoke outside :

Stapro do a very good flame retardant. There's one for 100% synthetic materials, and another for materials that contain ANY amount of natural fibre, even it's only small you should use this one.
Stapro don't give any warranty or guarantee for the product. What you can do though is with your invoice, provide a copy of the operator's manual for application, and write a covering letter saying that the correct product has been applied according to the manufacturer's instructions.
I've got a copy of that handbook if you need it

Good luck
