I have an order for a 50th anniversary cake. I know the woman who wants to order it and she is open to suggestions. The cake need to serve 75-100 people, and the kicker is that she will be driving 4 hours in July to bring the cake to the party. I live in Newfoundland, so the likelihood of humidity being a problem is low. I'm pretty sure it will be mostly highway driving. I'd be afraid of the possibility of it melting in the heat? What do you guys think? Should I just tell her it's not a good idea? thanks!

I think you should go over your concerns with your client, and see what she thinks. If she wants to go ahead with the order, make sure to have her sign something when she picks up the cake, saying that the cake is in good shape and the cake is no longer your responsibility once it enters the client's hands.

does she have a/c in the car?

Could you make one of the cool boxes that there was a discussion here about a few weeks (I think) ago to help it survive and use frosting, etc that can cope with heat.
That and a signed waiver from her I think you should be ok but I would suggest basing your decision once you have had discussions with her.