My fiance is being charged for a crime that he did not committe ?

Let me say this about that. If you think the shell casing is a plant, you might be right. But if it is, it's a long shot by the cops because without the gun or a bullet, they do not know the caliber at the time of the plant. If your guy really is innocent, his attorney will be able to help him. The chronology can be pinned down pretty closely. However, if all of this happened 3 months ago, and he has not been cleared yet, there's more to this than you're telling.