..a client wanted the cake for a party on Sunday, but actually wanted it to be delivered Friday evening since they would be out of town before the party?
It's a small cake for 10+ people and will be covered in fondant. They want a box mix yellow with buttercream. Would this hold up since I will be baking the cake Thursday evening?
I don't want to poison these folks come Sunday.
Any help is appreciated.

A yellow cake and buttercream willl be just fine. I'd probably put some moistening syrup on those layers to make sure they stay moist.

They are now wanting the cake covered in homemade buttercream instead of fondant.

That should be ok as long as they refridgerate it right?

I still think it will be OK with or without refrigeration. I did 2 big yellow cake sheetcakes for my sister's anniversary. Baked them ahead and froze them. Pulled them out on Wednesday, filled and iced them with homemade buttercream, and decorated them. Put them in boxes - they sat on the table until Friday when I loaded them in the car and drove them 250 miles to my sister's house. Served them Saturday and they were fine - my niece says they were still fine several days later.

Yes, yes, yes. There should be NO problem - especially if they keep the cake in a cool room temp location. Refrigerating it will hasten the drying out. As tigerhawk said, put it in a box and don't give it another thought

our moms and our grammas kept boxed cakes, covered in homemade icing, out on the counter for days and none of us died from it.

Thanks guys, I've never had to wait this long between baking and eating before, just wanted to get some info before actually doing this.