I took DSC_2618a at Crosby beach near Liverpool. If you look closely at the horizon where the wind turbines meet the sea, I think you can see the curvature of the earth. I took _DSC0994 in Amsterdam at the top of a high building. Here the curvature of the earth is easier to see... I just thought of another thing... seeing as 99.999% of the world believes the available evidence that the earth is a globe, then it is up to your friend to prove that the earth is flat.
Here is a photo I took recently at the seaside near Corby in the UK that shows the curvature of the earth... https://flic.kr/p/Vb6dqt I've also attached the photo... Look carefully at the horizon where the sea meets the sky. The furthest wind turbines seem to be beyond the brow of a hill... except that there is no hill because it's the sea and the sea has to be "flat". Therefore, what you're seeing is the "hill" created by the curvature of the earth! I'm not sure if the photo attached here will have enough resolution... probably best to go to flickr version.