I am doing a three teir cake this weekend for my boss's daughter. The party is 30 minutes away and I am way to scared to stack this cake ahead of time.
I told her i would be happy to go and set the cake up at the party but she said "that's okay. I'll just pick up the three cakes and set them up myself."
Yikes--this isn't a mr. potato head, it does take some practice to set up a larger cake. 
I say make her sign a disclaimer when she picks up the cake. That and take photos of it before.
Otherwise you can just tell her that you would rather drop off the cake and assemble it. Maybe say something about how stressed she must be with all this, and that you wouldnt want to add to her stress.
If it is anything over a 2 tier cake that is stacked (not where they just set it on plates for display) I will set it up. Personnel preference but to me presentation is everything.
If you are uncomfortable with her setting it up tell her that you set up all stacked cakes due to the construction of them.
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