It depends on whom you ask, and the book leaves the answer open on purpose. For Nastenka, the ending is happy: the lodger returns, in love, and the two marry the following week. For the Dreamer, it is a loss: he stays alone in the room, with fever, and sees himself there fifteen years from now, with the same maid. But he himself refuses to call the ending unhappy.
On the Fourth Night, for a few hours, he had everything: Nastenka accepted his hand, and the two of them made plans for a life together. A young man passed and called her by name; she ran to him, came back to kiss the Dreamer and went away with the other. In the Morning, her letter asks forgiveness, says she wished she could love both, and asks that he be a friend and brother.
His reply is not resentment. He refuses to cloud her happiness with a reproach and blesses her "for the minute of bliss and happiness that she gave to another heart, a lonely and grateful one". And he ends with a question: "A whole minute of bliss! Can that be too little, even for a whole human life?"
The final sentence allows two readings, and the text does not choose. In one, it is true gratitude: a man who spent his life dreaming has received a real minute and is content with it for the first time. In the other, it is the last reverie: he has just seen himself alone in the future and gives himself the noble role of the one who blesses, because that is a role he knows how to play. What weighs against the ironic reading is that, before knowing he would be left, he was already offering a love that did not demand to be preferred.