Yeah...that's not a good look no matter how you spin it. Too easy for people to notice and then start telling stories on DFP and elsewhere. Don't give them a reason.
I don't know. Kid may have had an appointment. May have been late for work. OP didn't know the kid, so he doesn't know either.
I look at it and say they had a kid for a weekend and into bracket play, so five, maybe six games. She played a couple of innings, had a few at bats. It doesn't appear they went out of their way to make her feel included or a part of the team -- by the original admission, she was just insurance in case an attending player couldn't play. Don't know what other promises were made, but if she just packed up her stuff, sounds like she didn't even have a jersey.
DD has guested for teams where she wasn't included in the team picture after the tourney, so I can understand a kid who doesn't feel used or included leaving, especially late in an elimination game. when she's been sparingly used all day. Like you all said, not a great look, but neither is recruiting a kid to just sit around for two days.