I have such an experience to share but not at Athens, rather at Larnaca. I was doing a segment run to complete the 4 required A3 segments for the lower Gold qualification threshold and ended up arriving at Larnaca at around 3am. I did have an afternoon flight back, so I had roughly 12 hours. I didn't want to go to a hotel as I would have to check out by 10-11am, so didn't seem like a good way to spend money, so decided to spend the entire stay at the lounges at the airport.

I had more trouble getting through the Transit area as everyone was puzzled to find out that I just came in from London and was going back without entering the country. However, Star Alliance rules allow for lounge access on day of travel, so I was confident I wouldn't have an issue getting into the lounge so many hours before my flight. I was already checked in and had a mobile boarding pass. When I finally got to the lounge, I was told that they close at 10am, so I had to leave for an hour or 2 until they reopen, however, I just used my Lounge Club membership and went to the other lounge across the hall, which worked out great as they had great reclining chairs that were quite comfortable.

So, long story short, you shouldn't have any issues staying 8 hours at the lounge on your day of travel, should you wish you to do so. Whether you want to, is another matter altogether. I also don't see an issue with you guesting your friend and then leaving to explore the city. I don't recall ever having to go through secondary checks once in the lounge.