They should get a survey history of the properly.
Is it a individual home ? or in a development ?
What was there BEFORE the house(s) were built ?
Are there any archives in the county library or county register to check on the history of the property ? to check and make sure it was not some kind of industry there before the house(s) were built ? Need to make sure it might not be old drums of ( what ever ) oil underground.
If it is truly a crude oil reserve under the properly, get the rights, permits, certificate for Mineral Rights as soon as possible.
Find a HONEST ( if that is possible in todays world ) lawyer.
They live in an area that was nothing but plum, peach, cherry, and fig orchards. When we moved to this street in 1972, one whole half of the street was still orchards. My parent's house was one of the first ones built (they purchased it from the original builder who lived in it for a number of years. It was built 1956, I think.
The area where the oil is seeping through had a fig tree and a walnut tree in it until a couple years ago. They just got too old, and died, and had to be cut down.
By the way, "The Front Porch" poster--do you mean "The Front Porch" in Los Gatos? I think that was a great store. Where did you move to?