Looks like you have big problem, once yr guppies tails are bitten, it takes a long while to grow. You have to make sure that medication is given so that there's no secondary infection from the wound. If no precautions are taken, then yr fish will die in a matter of days.

Your platy sounds like the victim but I also suspect he's not in good health which could be why yr guppy attacks only it. Guppies are usually very docile fish, frisky yes, but not to the point of aggression. Suggestion would be to medicate yr whole tank with natural remedy Melafix for 7 days (this helps with the tail and fin rot) to help yr fish grow back the tail. Prior to medication, do a 50% water change and take ALL yr fish out into a bucket filled with aquarium water. After the water change, dump them all back in the tank (its a tip I learnt that seems to help with aggression for oldies and newbies). Start medicating.

Hopefully in 7 days, you'll see improvement on the tail as well as a friendlier environment in yr tank.