Hi Neil
Have they attempted to remove it? If so, how? they may have fixed the stain if they have gone in with everything but the kitchen sink
Most of what you have mentioned above are drastic measures and not likely to help you solve the problem.
First rince out with cold water and blot, apply Solutions no 3 and rince out again, blot, repeat if neccesary. See how you get on, but be very careful to not over wet the rug.
Ian