Not one to be baffled by any science I'm not familiar with, I turned to Matt at Torq.
This is an interesting debate. There are actually lots of different Maltodextrins out there, but the one that is chosen by any sports nutritional company worth their salt is Maltodextrin DE 20. This means that the particular Maltodextrin has a dextrose equivalence of 20 (20 glucose molecules joined in a chain). The secret to Maltodextrin's success is in it's osmolality in the gut, because one molecule of DE 20 empties into the small intestine for absorption at the same rate as 1 molecule of glucose, but there's 20 times more glucose in it. It's also not sweet, so is pretty invisible to the pallet and can be used in quite high concentrations easily.
Your friend using apple juice is getting Fructose as a carbohydrate source, which has an extremely low Glycaemic Index. This means that it's very slow to get into the blood in a useable form and actually has to be processed by the liver before it can be metabolised.
The latest research has however shown that 2:1 Maltodextrin:Fructose works even better than Maltodextrin on its own (and infinitely better than fructose on its own), so perhaps you're both right! The research basically found that whilst maltodextrin competes with Glucose and Sucrose for absorption, Fructose has its own independent carrier, so doesn't interfere with the Maltodextrin uptake, so you're effectively getting a doubled effect – it works out to be about a 40% higher delivery of carbohydrate than Maltodextrin alone.
I hope this helps. Please see attached Performance Resource, you should find this an interesting read.
This is in a pdf format, so those who might like to know more, please email me and I'll pop it over on email.