It seems real. Atleast his part, but I don't think he meant literally marry her, he was just trying to motivate her, or maybe he said "if you lost weight it would be easier to find a boyfriend" and she exaggerated that. She probably exaggerated how nice he was, but the guy knew her since they were babies.  Now I'm assuming here, but most of his memories with her in them were when she was skinny, so he sees what was once his childhood friend killing herself and he's going to act on her as if she were still skinny. He sees the fat girl as having eaten someone he sees as a pretty, amazing, smart girl and is trying to reverse it through weight loss. That's my best guess and it's pretty sad if it's true.