Even if art is subjective, that doesn't justify claiming the art of others as your own.
Your friend may have made changes or gave the art a twist, but they did not make the original piece.
Lemme put it this way:
The Beatles wrote and made "Penny Lane" first.
Another singer/band does a cover of the song. They made a cover, made a new spin, but we all know the Beatles did the song first.
The Beatles own "Penny Lane" -- NOT the cover artist.
The same applies here.
Being a good friend means pointing out a friend can be in the wrong, even if you're not comfortable with that. You don't have to yell at them or be extreme, tho.
Just explain the why and how