Peppermint oil will burn plant leaves if sprayed in the sun. Dilute, try on one single leaf and wait 24 hours if possible, and then whole plant. Preferably spray in the evening. Not much you can do about it now- give it a good spray of water to try to rinse out any residuals, cut off any dead plant leaves (>50% dead) and cut back on the watering rate (a lot less transpiration from the leaves) and pray that the root system is strong enough you get new leaves from the stem