No, I understand completely, call me old fashioned, but I think any PIC, a wingman or not, has the responsibility of NOT hitting something.

When I did my training, the instructor did not have to tell me not to hit something.
Some things are given.
And avoiding a large, stationary, brightly colored piece of farm equipment would be a given to me.
This wasn't combat, not a fast jet aerobatic team where follow the leader is critical from a safety standpoint.
Thankfully, it wasn't the 1982 Thunderbirds crash.
Okay, let's play legalese...
We don't know how formal the formation flight was.
For all we know it was just two guys out having fun and one guy was in front.
If they had a formal preflight briefing saying "I'm the lead, you're my wingman", that would be one thing.
Did he take formal responsibility for the flight?
Was he aware he was supposed to be looking out for #2?
Do the FAA rules automatically make the guy in front the official/legal/responsible formation lead?
I would guess in absence of a officially stated
agreed upon formation, the lead figured the second pilot would look after himself.
I bet that's what the lead pilot's attorney is saying.

On top of whatever lead/wingman justification may or may not be out there, the fact remains they busted legal limits by flying too low to the occupied vehicle.
Per the FAA:
"An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure."
If the accident pilot uses the "I was only following...blame the other guy" defense, will he use the same line to excuse the busted altitude rule?
Would the FAA buy that?
Interesting stuff.