When you told her about this, was the point that you wanted to snicker at your in-laws for being silly, or because you thought what the ghostbusters found was frightening? I could see her not wanting to come to the shower for either reason, frankly. As RockRose suggests, don't stir up the waters by telling everyone; it will all sound a little over the top, and unfortunately you aren't immune because you would be the one passing the story around. You want someone to tell you that your friend is being a drama queen, but you're the one who has been dishing out the script.
So are you saying they called in the TV show Ghost Hunters? Or is this some spiritual group that may charge to "get rid" of the ghosts? Or not charge, but hold some kind of seance?
I wouldn't go either, I don't think. The whole thing seems unstable - your fiance's mom calling in some kind of hoodoo thing because she's worried about keeping your baby in her home and the harm that could come to it there from the ghosts.
And honestly, I don't think I'd tell any of the other guests either. Whether or not you believe in ghosts, this is a house where they're inviting oddness in and are themselves afraid of the spirits in their own home.
Hmm. Well, I don't know why they called in ghost hunters, and all of this recordings stuff could well be a scam to fool your inlaws, but if your friend (apparently along with your inlaws) is a person who is irrational about ghosts, that is just the way she is. It is not the end of the world if she does not come to your shower, she is still your friend. Showers are actually not the be-all and end-all of friendship. I had a friend who didn't come to my shower due to a sporting event, but she was the one who came at 1 am to sit with my son when I went to the hospital in an emergency. She's still your friend. Let it go.