If 2 opposing characters are adjacent and 1 is hovering other in grounded, can the hovering character use RCE on an attack and vice versa?

yes, the hovering character can use RCE, whether the opponent is adjacent or not. The grounded character can use RCE, but, only against the hovering character that he is adjacent to.

Ok I was told the Hovering character could use RCE but the grounded character couldn't, which made no sense at all to me.

Let's clarify this.

If there are only two figures adjacent, they are on opposing forces, and one (or both) are fliers, both can make ranged combat actions and attacks against the other. So, RCE will work for both in this situation.

If, however, a non-Flying figure is adjacent to *two* opposing figures, one which is Flying and one which is not, he *cannot* make ranged combat actions or attacks. *But* the Flying character who is adjacent to him *can*. Fliers can always make ranged combat attacks regardless of adjacency.