There is little point in speculating ask your friend.
- What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Have her check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections.
Also what type of scan was it she was doing when the detection was made ?
The first boot-time scan doesn't as far as I'm aware go as deep as an on-demand thorough scan with archives enabled. avast has also introduced new packer (archive unpackers) support so if there was an infected file in a previously unsupported archive (e.g. avast couldn't unpack it), now that it is supported it could find malware in a previously unsupported packer.
So hopefully you can see there may be many possible reasons why it wasn't previously detected, but again that is speculation and doesn't solve anything, we need hard information such as answers to the questions I asked about above.