I would not stay in the lounge for so long. 8 hours is enough to go into the city by taxi, train or bus and enjoy yourself there for some hours. You'll also be able to find a much nicer lunch - even if that means a 2 euro pita: Beats the food at any lounge!

One other thing I can recommend: I went up mount Lycabettus last week (tip: take a taxi up for about 3.50 euro, walk down afterwards) and we had some great drinks at the restaurant/bar up there. The views are breathtaking - and if your friend likes to take it easy - that would be a great place to do so. Also very affordable (coffee/soda drinks for less than 4 euro) which is very good at such location.