Dont tell her the name, dont give her the original container! Just tell her how to use it ir give her a couple of litres pre-diluted. Write down a few lines of direcion on how to use it if you have too.
I'm not sure of the dilution rate for tiles...
I agree with Mike tho, give it to her for free. A couple of litres wont break the bank. Tell her the 5 litres costs you £50, but you buy it in bulk and NOT avialble to the public!
Tony