Not an expert but think it's a long shot to find a free message app that will work with an old iPhone and iOS 4.
But has she set the iPhone up with iTunes and checked that she has got the latest available iOS for her phone? A slim chance that, if she then goes into the app store through the iPhone, she might find a message app. It seems that about a year ago Apple allowed previous versions of apps to be available but I'm not sure if you have to already have the app on your device.
Also, you don't mention where she is travelling but she does realise that the 'free' message apps require data/wifi? Sorry if I'm telling you what's already known! |