I have a cake being picked up fri arfternoon. I'm out all day tomorrow. So I have had to bake, split, fill and ganache tonight (wed night). I won't be able to cover in fondant and decorate til fri morn.
I usually ganache, chill for 1/2-1hr, then fondant.

Question is - should I fridge it or freeze it? And how long will it take for the core to come back to room temp for me to fondant it? It's an 8inch square, choc mud.
TIA

Freeze. When you get home Thursday night, remove from freezer so that by the time you wake on Friday it'll be at a temp you can work with.

I don't understand why you would need to freeze it? Ganache is fine in the fridge for over a week, your cake won't dry out after 2 days as the ganache seals in the moisture. I'd just fridge it, but ultimately it's up to you.