The tip about her using WiFi somewhere is the key. Your iPhones came already with both an instant messenger program called "Messages," and a video conference program called "FaceTime." They both work great, no need to download anything. And regardless of carrier, using Wifi solves the problem.
(Before anybody whines about my assumption that they both had iPhones, I must point out that the OP didn't say what phones were involved, so all I can do is assume -- either that they have Apple phones, or that they don't. I made my choice.)