I'll echo what everyone else says: just give him time. And don't stress about it, or the cat will pick that up and feel more stressed. Where is he hiding? If it's somewhere that you can hang out close to (ie: cat's under the bed, you can sit on the bed; cat's under the sofa, you can sit in a chair across from the sofa) just settle in, read a book or watch a movie, and talk sweet talk to the kitty once in a while. Sprinkling some catnip in a more open area near his hiding place (ie: 6 inches away from the sofa, if he's hiding under the sofa, but not yet in the middle of the room, which takes him far from his current "safe place), can coax him out (unless he's one of the percentage of cats who aren't affected by catnip). And when he does come out, just talk to him, tell him he's a good boy, use his name. Try to let him come to you, even though you may be tempted to go and grab him.
I had talked a neighbor of mine into adopting one of the cats at the shelter who was very shy, but very sweet. I hadn't seen the neighbor for three weeks after the adoption (our schedules are very different), and when I did and asked her how the cat was she said "o.k., I guess, I haven't seen her." Seems that for those three weeks, the cat hid all day, and hung out with her other cat at night (doing her business and eating, but otherwise invisible). We went down in the basement where the cat was (but neighbor didn't know where), and I spotted a perfect hiding place on top of some old mattresses. We looked, and there she was. We took her upstairs and let her roam whereever she wanted (her choice was to hid in the piano!). Little by little, she got braver, and eventually became very friendly (though still hid from stangers).
If this goes on for too long (like three weeks), then you might want to pry kitty out of hiding, having some catnip and toys and lots of pets ready so kitty remembers the good things and not the "prying". Also, ask your friend what special things kitty especially likes (toys, treats).
I adopted my most recent addition, Rascal, because even though he is young, and healthy, and very sweet (never a mean gesture to human or cat), he is very much a scaredy cat and wouldn't socialize with anyone (except me, after he got to know me). They didn't think he'd adapt well to moving into a different room at the shelter ( they were going to move the "adolescents" into the adult room to make room for new kittens in the kitten room and put the current batch of kittens in the ado room), and were seriously thinking of euthanizing him. When I brought him home, he hid in a corner (he was too scared to even try the basement!) for a day (craning his neck and looking like a Komodo dragon to look around at all the scarey things). Then came out into a cat tunnel for about two days: startling every time I walked by. Then he hung out under the sofa for about 3 days. Now, he's much more social and doesn't hide (but he still gets scared of new experiences and sometimes if I make quick gestures---though he's never been hit or mistreated). He'll come on the bed when I go to bed, and in the morning and stretch out sooo looooong, but he's only been staying about 3 minutes at most (he has feline ADD, I think!). In the last couple of days, he's actually stayed for about 1/2 hour when I took a nap, and 15 minutes at night.
Please do give him some time. Someone adopted a cat from the shelter, got frustrated after only one week when he wouldn't come out of the closet, and brought him back.
