Just how....

[Kaban]

I want to say the cost of that car is not worth the cost of the detail that is needed to restore it (if restoring it is even possible).

Would be "cool" to use it as a test car to try out different products and tools though.

[rolop6]

First off, that's just nasty. You don't know how many times I've done extra stuff at no charge thinking people would run to everyone they know telling them how great I am.
If I were you, don't count on that. People typically remember when you do bad, not when you do good.
With that said, for me it would be a $400 detail, if money isn't an issue for you you should do it for free and write it off.
It is doable but you'll need to use steam and a good extractor.
Take seats out, pressure wash them with a good upholstery cleaner, take carpet out and pressure wash it. More than likely it will not get 100% clean buy about 4 cans of carpet and upholstery dye. Dye it and it should look like new.