How do you know she was new, how do you know she was express, how do you know she was a standby (over positive space), etc?
I'm sure there's some written policy about it somewhere but in the big picture of life, did that really affect much? Whole meal, yeah, that'd be a big deal. If it was me, rather than risking getting someone in trouble and possibly facing job action without being totally sure of the situation, next time (and hopefully it doesn't happen again!) maybe just sarcastically say something to your seat mate like "gee I paid a lot of money for this seat, hopefully there's nothing else on the flight I'll have to skip. Are there any other allergies we should tell the flight attendant about now?" If she's actually a standby, she'll probably get the hint to not do it again.