Yep, first-hand knowledge of a committed crime can lead to obstruction of justice or accessory charges, depending on the mood of the DA. They can throw a few more on top too, just depends on the gaming laws specific to each state.
Get on the horn and blow the whistle, on both them and your "friend".
Your "friend" should have called to tell you the good news---about how the guys that poached that monster buck you saw were taken away in cuffs...not how impressive the rack or the stats of the kill.
Or at least he should have called to tell you how good it felt to slug the damn poacher up side the head <img src=icon_smile_wink.gif border=0 align=middle> twice

, once for him and once for you <img src=icon_smile_big.gif border=0 align=middle>