I can't think of many Metal songs playable on an acoustic. You could try some Black Label Society, they make the most tear-wrenching ballads I've heard... We still have the problem of electric guitar.
Maybe... If you think you could play the piano chords on acoustic? I have no idea how that would sound, but I think it's all I can contribute. Of course, I don't know just how sad you want this goodbye to be, if you want it to be at least a little happy maybe stay away from Black Label...
Ozzy might have a song you'd find suitable... Nothing immediately comes to mind, but he does a lot of beautiful songs. Again, guitar is often a bit too complex or not-clean for an acoustic...