Has your father-in-law considered going to a different mechanic. The problem is more than just a change of spark plug. Has the mechanic considered anything else? Are you using Motorcraft plugs? Rangers seem partial to them. Other brands seem to cause problems. Are any other trouble codes displayed? What brand of fuel is used? Have any of the other suggestions been checked out? The mechanic seems to only be addressing the obvious problem but is not digging deeper to identify and correct the root problem.

Keep us posted on the progress.