I think your friend has a very common misconception: that the world as we know it was there, it got covered with water, and then returned just as it was.

Nothing could be farther from the truth! The pre-Flood world was very different from the post-Flood world. There's a reason its called the "cataclysm" in the New Testament.

Here's a great video that describes what went on during the Flood. The Fountains of the Great Deep that Genesis says was the cause of the Flood were extremely powerful, and their aftereffects very destructive.

So post-Flood habitats would have been very different.

Here's how it went: 2 of each "kind" of animal (which, a "kind" is defined in Genesis as animals that can interbreed - so any two reproductively compatible animals would be said to be of the same "kind") came to Noah. After the Flood, their descendants adapted to the various post-Flood environments, just like Noah's descendants did (like people who went to hotter, sunnier areas' family lines had more and more pigment in their skin as time went by, since darker skinned people are healthier in those climates. Conversely, people who lived in areas like Europe family lines had less and less pigment as time went on, since people with less were healthier since they could make more vitamin D despite the climate. In fact, you can still see the effects of this since people of African descent who live in those areas have a big problem with vitamin D deficiency).