If no parasites, or bacterial growths are observed on the guppy, and no aggression was seen between the two fish, then the guppy simply might not be strong enough to go through the cycling process.
If you think the guppy has a fighting chance, another partial WC
might help. Probably a good idea to buy a few more Platys, Mollies or Zebra Danios to cycle the tank.
A good water conditioner helps during cycling. It seems most forum members, including me, use "Prime". Hope things level out soon.