Yes, that's correct.
If your friend is on a server with a large hard drive, dual CPUs, and not too many users - she will effectively be able to use more CPU resources and have a busier site (both in terms of CPU usage and traffic) than she would if she were on a less robust and / or more crowded server.
Even if she's on a hosting plan that allows her a large amount of bandwidth, if she exceeds a certain percentage of CPU usage, the host may ask her to upgrade to a dedicated server - though ideally that shouldn't be the host's first method used to deal with the problem. If the host can just move your friend's account to a less crowded server, that would let your friend keep the same hosting plan and not pay more while her host's other customers would not (presumably) have the negative impact of a high-traffic/resource-intensive site slowing down the server for everyone.
Keep in mind, though, that if she's using 5% of the CPU resources (just to pick a random number) on an occasional, short-term basis - the host still may ask her to upgrade if that 5% is regular enough, and of a long enough duration, as to significantly impact the other customers' sites' performance. The "red-flag" percentage may vary from host to host, and even possibly from server to server.