Basically yeast ferments the sugars and produces alcohol and CO2.
On the most basic level - more fermentable sugar = more alcohol. This is only within a certain range - part of the reason being that yeast cannot tolerate too high an alcoholic environment so essentially they kill themselves. However just wacking in a shitload of basic sugar will make horrible bathtub beer and you may as well drink meths (cheaper, higher alcohol and less effort).
Additionally basic sugars dry and thin out the beer. Something like malt or malt extract will add to the body of the beer as body is related to unfermentable sugars and malt is only partially fermentable. Thus malt can be used to give more abv and maintain/increase the body. However the unfermentable sugars leave residual sweetness which may be balanced out with bittering hops and/or by drying out (with simple sugars for instance).
That (lots of malt + good yeast + simple sugars) is how high alcohol Belgian beer is made and prevented from becoming cloying. You can increase the ABV of your brews in a number of ways but once you start getting past around 6% (arbritrary figure so don't quote me exactly) it becomes much more of a balancing act and you need to treat the yeast well and ferment processes need to be schmik. People can make beers 18+% but they are usually complicated cellerable barley wines and not simple. (http://en.wikipedia.org/wiki/Dogfish_Head_Brewery)
An easy way if you are a KK brewer is to either add some extra sugar (preferably in the form of dried malt or liquid malt) or to make a toucan (two tins of whatever kit you use + yeast and amount of water to make only one kit). Otherwise you can drop the amount of water (say 18 L instead of 23) or a bit of all 3.
You can also add extra dextrose, extra malt and balance it with maltodextrin which adds body but no flavour or alcohol. Myself I'd recommend either the toucan method or extra malt and doing a boil with bittering hops as they will make better beer than dex, malt and maltodextrin. It depends how far you want to take it. Remember to balance flavour, body and bitterness if you want to avoid your friend's mistakes.
There is a thread on toucans containing loads of recipes as well as an article in the wiki section.
