I wouldn't even worry about confirmed standby. (Save your $25 to put towards a rental car...) Looking at the flights, there's plenty of availability on all of the LAX-DFW flights this week. There's also a back up plan to catch 2408 at 12:40 AM if the 2404 at 12:15 doesn't pan out. Both can connect in DFW to EWR on the 6:30 AM flight.

No available seats showing on the Monday morning DFW-EWR flight, so that could get dicey. But on the 6th and 7th there are 45+ and 30+ seats respectively.

I would just rent a car. Depending where she needs to go, it'll be much easier. Plus if she just needs to be there for the day, she can bring it right back to EWR and then get into NYC to meet up with you. I'm very familiar with the area. if you need some help, let me know.