The premise of the story is really interesting: Can you allow yourself to love someone that someone dear to you, who's passed away, also loved dearly?
The thought of that question has honestly never even crossed my mind once, however, after reading this I've come to the conclusion that "love conquers all" - if both parties love each other, the unfulfilled desires of a deceased person shouldn't matter. Tougo loved Natsumi all along - not Haru; if Haru had been cured of cancer, and, thus, married Tougo, she'd have become increasingly jealous of Natsumi due to Tougo's growing feelings for Natsumi. There's no doubt, it wouldn't have been good for Tougo either. I think they made the right choice; even if Haru had survived the illness, I think she should have chosen to let go of Tougo. He loved Natsumi all along and she knew it.
Haru No Noroi