I read that a devotee was told to feed ants. Who was this devotee and why did he do it?

The devotee you are referring to is Kashiram. According to Chapter 0, Kashiram carried a bundle of powdered sugar with him specifically for the purpose of feeding ants. This practice was not directly instructed by Maharaj, but was advice he had received from another saint named Janakidas Baba. Kashiram held this practice so dear that when robbers tried to steal his bundle of sugar, he fought back fiercely, risking his own life to protect it. This demonstrates his commitment to the saint's advice and his compassionate nature.