Can someone suggest me guitar strings..?

The rattling noise might also be because the action is too high or the strings are a bit loose, but you might be better off changing your strings first and seeing if that improves it. Especially if you haven't changed them in 8 years! By the way, all guitars have six strings, except bass guitars which have four (or in some cases five, but those are more unusual.) What kind of guitar is it? Classical or a dreadnought? Or, to ask another way, are the strings on it currently nylon or steel?

My favourite steel strings are Elixir, they're really smooth and you don't even need to change them THAT often. Another good option is D'Addario. Beyond that, a lot of guitar manufacturers also make strings, like Fender and Gibson, but I still prefer Elixir or D'Addario.

The gauge (thickness) of the strings is also important - if you're trying to toss up between a lighter (smaller) gauge or a larger one, it's probably best to go for a lighter gauge if your girlfriend is a beginner - lighter gauges are easier to manipulate.

Do you know how to change strings? Might be worth getting a string winder, it'll make life easier. A lot of music stores can do it for you though. If you're really stuck I highly recommend throwing yourself on the mercy of your local music store, those guys answer questions about guitar strings all day long, so they know what they're talking about.

Good luck, maybe when you get back she'll be a kickass guitarist!