The first thing I would do is review the regulations and if there are stipulations for handicapped people.
Sometimes officers take things at their own discretion; kinda like an officer deciding to give a ticket or a warning for speeding.
Your friend first should consider that maybe the first set of officers were making an exception for him and the 2nd officer was going by the book. If there are rules saying people can't access certain spots via atv and doesn't make reference for the handicapped with special permits, then your friend should be more considerate for the freedoms he has had thus far and realize he won't get far in his case proceedings.
I think if special favor was personally granted for his atv use without permit, his self-centeredness should be more of gratitude than a willingness to take the 2nd officer to court.
If no such special permit exists for the handicapped, then your friend, if he cares enough, can take matters to the legislature and get such a law applied. The DNR could also maybe point him in the right direction to take.
Good luck,
iSnipe