Through Eris, Arad could sense a bit of what happened. He did feel her distress, worry, and eventual relief. Gojo, who could directly communicate with Lucy through his dreams, came to Arad's in the middle of the night to inform him of what happened.
How Greed took over Mira, and how she came back safely after having resisted the portfolio's drive, and how her father had become a demigod, but still hasn't fully recovered. Even though he gained too much power, his body and mind would still need some time to adapt and acclimate.
But the part that made Arad mad was when Gojo told Arad that it all started because Amaterasu triggered Mira's greed. Amaterasu did later explain to Lucy that if left untended too, Greed would've slowly taken over Mira's mind, so someone had to bring it to light. But that conversation happened after Lucy already told Gojo everything.
