Do you want to re-record or just boost the volume in the actual record?
Because those two are different.
Re-recording is "simpler" if possible, but boosting the volume of an audio file results in quality loss.
Does your friend still have the original mix with all the tracks, where the voice is separate from the music, because that would work better than boosting the volume of a mixed down audio clip?
You can use Garage Band or maybe the software that was used in the first place.
Do you have any more information?