Globe Autoload and Smart eLoad can be bought here and loaded remotely to the Philippines phone.
I have used http://www.phonecards.moonfruit.com/ for many years now, occasionally a couple of hicups but generally they are very good, it's not cheap though, five pounds for 300 peso load.
Problem is she will still be paying 20 peso a text to txt you here. Also if you have a roaming Philippines SIM here you will be paying heavily to reply although she will get cheap txt to you.
If you have a laptop you can buy a Smart Plug n Talk device which will give you 1 peso txt in both directions, the major inconvenience being that you have to have your laptop open all the time.
http://filipinaroses.com/sh...ad.php?t=18482
Jim