Hello Cindy - By 'Greek Guitar' do you mean a bouzouki? It is a long-necked lute-like instrument with paired strings - usually 4 or 5 pairs (8 or 10 strings). It is kind of like a mandolin on steroids - sort of like an oud (turkish lute) with frets.
Bouzoukis are sometimes used in traditional Irish music sort of like a ryhthm guitar. they are also (obviously) used for Greek folk music. There are several websites out there with tunings, chords, tips, etc. Go to Wikipedia and search for 'bouzouki' and see the links at the bottom of the article.
Does your friend already play guitar or mandolin? If so, a bouzouki would be a great gift. Be wary of cheapo souvineer-type instruments that might look nice hanging on the wall, but that don't play well and don't sound good.
"That darn Pythagorean Comma thing keeps messing me up!"