On Killing by Dave Grossman explores the research backing up your friend's assertion in depth. I highly recommend that book to anyone whose interest is piqued by your comment.
[1] From https://www.gwern.net/docs/history/s-l-a-marshall/1988-spill...:
> His surviving field notebooks show no signs of statistical compilations that would have been necessary to deduce a ratio as precise as Marshall reported later in "Men Against Fire".
For those still reading, the controversial aspects that grandmczeb speaks of (and which Grossman has addressed) is discussed here: