She should stay in...
...and watch more films. She might have been spared her monkeygeddon experience:
http://www.youtube.com/watch?v=h3jak1wfO40
2 publicly visible posts • joined 30 Jul 2009
I don't think you'll be able to get the GPS Pursuit idea to work that well. In the UK we're not allowed to send up a decent RF transmitter on a balloon (the only real option being a 10mW module using one of the unlicensed bands i.e. ~433MHz). Using a mobile phone you'll probably run into reception problems past 1000 feet.
Your best option is probably to combine both options though and have the RF module transmitting the GPS co-ordinates, or a simple beacon signal at a low data rate. Then have the mobile phone also send the GPS co-ordinates via SMS when the payload comes down.
If you get the GPS data via SMS, then you can also use the RF beacon along with a directional antenna to pinpoint the payload.