You know, when I saw this thread title, I thought it was going to be about the size of the invite list! 
I've been with guys on both end of the spectrums. One guy was huge, about 9 inches, and every time we had sex it was like, "Ow! My kidneys!" There was another guy who was hung like a baby carrot and I could never tell if he was in or not (Note: Guys don't like it if you ask that.) Needless to say, both weren't that good in bed b/c they had no other techniques nor were they interested in trying. As most other women have said, skill makes up for size.
If your friend's guy is that great, there are other things that can be done to compensate. There are some sex toys that can be strapped onto the penis with vibrating heads and they add a few inches. Plus, one of the benefits of being with a smaller guy is that it's easier to perform oral sex and some positions are easier.
I won't lie...I like anything in the 5-7 inch range. It's nice to have some substance, but ultimately personality is much more important. Ask your friend this--would she be mad if a guy broke up with her because of her breast size?
