Assuming all of the music is acquired legally through perhaps the Music/iTunes Store for that phone owner who hasn't previously purchased a Mac, when they do buy that Mac, they will tie it to the same iTunes Store ID and the music will download to the Mac. If they've recorded some of their own music on the phone, when they sync the phone with that Mac, it should sync over.
And ASAP after you resolve this situation, I encourage you to get your lone source of media synched up with a Mac (or PC) and get a Time Machine disc or two attached to that Mac (or equivalent to PC) so you have backups of what is on the computer and what is on that phone.
As is, you at least have ONE device with your personal media on it. What if you lost that device? Stolen? Destroyed? Your post reads like you would have NO recovery at all. Trusting it all on any one device is begging to lose it all at any random point in time... for all kinds of possible reasons.