thanks CB. I basically just copied and pasted (tweaked a bit) what you said above. I hope she won't ignore me. If she does though, should I just let go? Or should I keep pursuing this? Is there even anything I can do? I just want to calm down and not have to worry or think about this anymore. You have helped loads as you always do with all my problems . You're a gem!
Unless you can figure out where he's been rehomed to, I don't think you have a choice but to let it go. You could stalk all her FB friends, I suppose, and hope that if you find the dog via pictures whoever has him won't mind you coming across like a crazy stalker person. That's assuming that their account is public, etc.
Best case scenario - you find the pup, he's alive, and maybe he needs a new home. Can you give him one? Can you give his owner a few hundred quid to help pay for food/vet bills/whatever? If so, definitely pursue it. If not, I'd just try and let it go (unless you wanted to try and get the RSPCA involved or something).
Worst case scenario - you find out he did die, or got rehomed someplace awful, or both, and can't do anything about it. Then you're left feeling really bad, still.
I know it's really hard not knowing all the information, but if this woman ends up being stubborn, and/or FB stalking doesn't work, you won't be able to find out anything anyway.
Good luck!