Illegal?

[MuskieBait]

Why knife? A nail cutter is a much easier tool to use and you can clip 100lb mono just as easy. I even used nail cutters to cut braid up to 65lbs. No issues at all.

I can see the need for a bait knife though.

I do agree with Mike. Your cousin must have a crappy lawyer. A reasonable knife can be carried if you're partaking outdoor recreational activities like fishing, hunting, camping and even scuba diving. Unless the officer caught your cousin with a knife and no outdoor gear, the officer may have a case.