Is your friend the inventor of the Fuel Doctor??

If so, than I'd say it is working quite well for its intended purpose.

It is intended to use pseudoscience (look it up) to baffle you into thinking that this can work and get you to part with a pretty hefty chunk of your hard earned money for something he pays 50¢ for from China.
I can GUARANTEE, that if you could get the honest truth out of the guy who invented this thing, he'd tell you himself, it is a gimmick to try and make himself rich.
No different than the Intake Tornado, Fuel Line Magnetizers, Water Ionizers for your drinking water, Homeopathic Remedies, LED Light therapy devices, HHO Generators and Snake Oil. All these products do is take your money and give nothing they claim in return.
In all seriousness, you are an unabridged sucker if you bought this thing. Learn from it, and be more skeptical in the future. I have lost prolly $5,000 of my own money over the years being suckered myself on various deals.

It happens.

If you only spend $50-60 on this thing and endeavor to keep yourself better informed with your purchases and business dealings in the future, and somehow don’t get duped again, than the Fuel Doctor is a bargain. Of course it's not the bargain for which it is intended to be. It does have a 30 day money back guarantee, get the refund.
Pax et Lux