Why would she do that, he asks. Playing around about Santa is fun and all, but this was real.
Mai is crushed. Why would he say something so cruel?
And since we're speedrunning the anime tropes, as she runs off she admits that she fell in love with him.
Santa goes to the party at his senpai's house, but she denies ever calling.
But hey, there's a party with friends, come on in.
But Santa puts two and two together: Mai used her magic to help him, and he dropped her like a hot rock once he had a better opportunity.