They didn't forget to feed him, it's just that the author is implying that the only reason Koromaru is sad Aragaki died is because now he won't get any more high class dog food (rather than, you know...because he'll never see his friend again)
:D... Don't worry Koromaru... I'll feed you my leftover highclass cat food! *thumbs up*
*dog whine*He really liked Senpai, huh...Koro-chan, what are you staring at...*stare*high class dog food (empty)*empty*Koromaru-san says, "If only I had asked for mo-No need to translate!!We pretty much get the rough idea!*dog whine*Ever since Aragaki-senpai died, he always seems so down.There, there.