I've heard a lot of different things about this. I eat Paleo/Primal, which from what I understand is similar to SB in it's limitation of sugar and carbs. A lot of people eat a sweet potato as a "recovery" food. (I hope you don't mind I jumped in here - I *hate* it when people tell me to carb load!)
You could experiment; See how it changes how you feel, your mental alertness, your results. I always end up feeling tired after a large consumption of carbs. And I did my best running when I was on a limited carb diet.
Ultimately though, WHY are you doing the 5k? Is it to "win" or for competition? If you're doing this for your own personal best, I think you should do it YOUR way. Your friend means well