This looks slightly different to what my fish got a while ago. with mine it was confined to the edges of the fin. Both of mine were also female platies, so same species treatment which helps.
I successfully treated something looking VERY similar using salt. I had two fish go down. I didn't start proper treatment on one until it was too late and I lost her, but the other made a full recovery. The damaged parts of her fins fell off and regrew.
Isolate the fish in a hospital tank because this could be contagious. Put two teaspoons of salt per gallon of water in the hospital tank. Get a clean bucket and fill it 3/4 with treated water. Add aquarium salt. You will need quite a lot. Buy or borrow a hydrometer (instrument for measuring water salinity) and keep adding salt until the water is at the bottom end of the seawater salinity range. I don't remember how much salt this actually took because i was in a panic and I just kept tipping in salt until the needle went up.
Put a heater in the bucket and keep the temperature of the salt water identical to the water in the hospital tank. I reduced the temperature in both to a few degrees colder than the main tank which did seem to slow the advancement of the finrot. This also keeps the oxygen levels up in the salt water. Three or four times a day put the fish in the salty water for 20 minutes to half an hour. Platies can cope with this much salt for short periods, they are sometimes used to cycle marine tanks or even kept there permanently. Salt water contains less oxygen than fresh water, so run as many airstones as you can find at full pressure in the bucket when the fish is in there. If she shows any signs of distress, take her out and put her back in the slightly brackish water in the hospital tank.
This may be too advanced to treat. See how at the bottom edge of her fin the white is almost touching her flesh? It's worth a try though, I did manage to save a fish that looked almost this bad.
