If they depart IFR and enter the NAS, you can accomplish this via FlightAware Alerts.
I agree with Jason in that this was probably done with FlightAware. For free FlightAware allows you to track an aircraft and have alerts sent to both email and cellphone. My wife and other family members (via email forwarding, since only one email address is allowed) use this to track my progress when I fly back and forth between White Plains and NY.
For IFR flights, FlightAware will send an alert when the IFR flight plan has been filed (within 6 hours of estimated departure), when the flight has departed, if there is a diversion, and when it lands.
Here is a sample email showing a flight landing:
The text version of this message is a bit less verbose. FlightAware will do VFR flights, too, depending on if the VFR flight is receiving VFR flight following.
edit: In the case of VFR flights, all alerts except filed IFR flight plan are sent.