A week is a really short time to decide! I guess it depends a lot on the company, I thought 3-4 weeks was normal for most graduate schemes.
When I asked for an extension PwC were fine about it. I got the impression that an extension of a couple of weeks wouldn’t have been a problem at all. As I was asking for quite a long extension they gave me the extension but did warn me that if I left it too late, I might need to choose another service line or office. I applied to a regional office where they only take on about 3 graduates in my service line, so basicly if those 3 places filled up before I’d accepted the offer, I would have had to choose something else. I applied early though (June 08 for sep 09 start) so I was confident that I had a couple of months before all the positions would start to go. If I had applied later, I might not have been willing to take the risk of the extension (and maybe PwC wouldn’t have been willing to give it).
If your friend wants more time to decide she should conisider calling the company and asking them. As long as she is polite then even if they don’t give an extension it shouldn’t have any negative effects.