Well in that case make it clear that it is an "attempted" stain removal and give no guarantees. My concern would be that the oil content has gone right down through the carpet into the backing, underlay and maybe even the subfloor.
I would be bordering on the thought that it might not even be ethical to attempt, knowing this. I tell customers that if I don't consider the chances being any better than 50/50 I don't even recommend attempting it, but can provide an insurance report.
It is sometimes an in-road to further work but not always - you have to be a good judge to decide if it's worth your time and effort or not. I've gained many good paying customers from putting myself out to help them in these cases, but equally have made no money and had my time selfishly consumed for no further benefit to myself.
Your call
