Good way to fix water damaged windowsill?

Hello there,

Family member had a plant in a jar on the windowsill over a longer period of time and over time (i guess while watering) water spillage has made its way under the jar and made quite the damage to the sill. Is there a good way to fix such a damage? Will obviously from now on have something underneath the jars to prevent this in the future.

Appreciate any tips!