After using and loving my 64GB iPhone 4S I could not mentally or physically go back to using a 32Gb iPhone 3GS. My mind would feel like it was in "Groundhog Day" my fingers would curl backwards like the Wicked Witch's Toes in The Wizard of Oz.
Don't do it. Tell your friend, you want to stay friends and decline the offer. Help him or her raise $ to buy their own iPhone 4S 64Gb.