That was just bad writing(or the networked forced a happy ending, I don't know). The Sunnydale-denial can only explain so much, I mean, Willow & Xander lose a friend, who Xander killed, yet the end of the episode have a happy endning and Jesse's never mentioned again.
I had the feeling they had known each other for a while, and even if they didn't, that should still hurt. Just see how everyone reacts to Cassie's death in season 7, yet they barely knew her. That's good writing and it feels realistic.