"However, he feels like he owes Mari-san because apparently she died thinking Kuroiwa held a grudge against her"
This is wrong, sorry, I'm very stupid. Asako actually thinks he should tell Kuroiwa about the dreams because Kuroiwa wrongfully thinks Mari died holding a grudge against him, so reassuring him that Mari actually loved him until the end could help relieve Kuroiwa's conscience.