First of all, you're seeking legal advice on a web board. Hire an attorney. A few attorney's visit this site, none can help you here, though you may still get some good unofficial counsel.
If you work for the airport, the airport should already have an attorney on retainer. If it's a municipality, most certainly the municipality had better have an attorney on retainer. It's usually a city and county office.
Second of all, not nearly enough information has been provided, as evidenced by the fact that each respondant needs to ask questions in order to answer the question.
Did the pilot in question (PIQ) have authorization to land on the grass strip?
Is the grass strip a runway?
Is the grass strip commonly used as a runway.
Is the PIQ local?
What is the relationship between the PIQ and the airport?
Who had a duty to ensure the strip was free of obstacles?
Does the entity responsible for the field normally ensure that the strip is obstacle free and clear of weeds or brush?
Is the strip marked or listed in any publication as a runway or designated landing area?
Is the grass strip physically marked as a runway or takeoff/landing area?
Are any signs, placards, pamphlets, publications or is there any other information detailing that strip as a landing area?
Does the airport attempt to warn of hazards on the strip?
Is the airport insured? If so, what does the insurance company have to say about this strip?
You stated "us," in describing the entity or organization attempting to disprove this pilot's claim. Who is "us," and how do you fit in?
If in deed you are part of the "us" that is attempting to disprove the pilot's claim, and you are aware of the potential NTSB report, why have you not located it and read it?
You stated that a runway marker was located on the strip, but did not elaborate. Please do so.
You also stated that the runway marker was obscured by tall fauna. Elaborate on that, also. Who had a duty to keep that marker clear?
Did the incident take place in daylight, or at night?
Is the runway available for night landings?
What type aircraft was involved? Some aircraft have much better forward visibility than others...some have little or none.
What was the landing pilot's experience level?
Have you looked in the regulations at all, or are you hoping someone else will do it for you?
Has legal action already been filed? If so, at what stage of litigation is this matter presently?
With respect to your feelings toward landing on grass, don't let your own inexperience in that area, or general prejudice, attempt to determine the validity of the other pilot's actions. Grass runways are a perfectly legitimate source, and if I'm paying for the tires, I'll certainly choose grass over asphalt for longevity. Tires used on grass only will all but last forever...you'll change them for age before you'll change them for wear. Not so for landings on asphalt, macadam, tarmac, cement, whatever.