Mark, slightly naughty but even so I'd still use that, but only as plan B!
Plan A is to explain to the customer that Scotchgard is widely used as a generic term for Protector. Throw the word Fluorocarbon at her and watch the blank expression
You can sell her whatever protector you want to, as long as it's a good quality one, and you can win her over by explaining that it's exactly the same thing as Scotchgard but cheaper.
I think in situations like this you can't let customers start calling the shots because they don't know what the bloody hell they're talking about most of the time! You have to take control and steer them, because you're the expert.
What does she want you to guarantee? That her kids won't throw up on the carpet?!! That nobody will ever get it dirty?
I'd set things out very clearly that a protector will increase liquid repellance, slow down the rate that soil gets ground in and make carpet cleaning more effective in the future. It is NOT a magic forcefield and some stains WILL defeat it, but over the years a protected carpet will look better and be more free from spots than an un-protected carpet.
Maybe she's getting mixed up with the Guardsman warranty? Basically an insurance policy, probably meaning she's double insured unless she doesn't have home insurance! Cheaper to protect the carpet and if needed get you out to spot clean than pay all those premiums.