Aww she’s happy, I can get why she might have felt jealous. But he already turned her invitation away last time, he said it himself that he values the time the two of them have together, and he intends to uphold his promise of keeping that day just for the two of them.
She’s a nice girl don’t get me wrong, but he just doesn’t see her that way, at least not right now, who knows about the future. He’s just happy to have a new friend, someone who he might not share interest with, but someone who is interested in him as a person. That’s valuable in its own right.