I am making a cake for a friend that is traveling 6 hours with cake should I put support in the cake to prevent it from sliding? It's a 3 layer caker (single 10" cake).

Nope. Secure it to the board with icing. I personally would box it and then I have a big square cooler that I take when I'm bringing cakes on a road trip. I've brought cakes up to 9 hours away like this.

No support needed. I typically will use double stick tape to connect the cake board to the box to make sure it does not move during transporting.

Additionally, I agree with aligotmatt too.