After going through chemo with two Collies I think your friend made the best decision. I was at K-State every week for treatment for Strider, (if you do a search you can read the story from discovery to end of life).
I am convinced it has more to do with the gene's then outside influence, my opinion.
And you are correct some breeds are more susceptible then others. One lady had 2 Goldens that had cancer at the same time.
My opinion is subject to change as treatment advances.
I just lost my Bud boy at the end of January to what they believe to be a brain tumor, don't know if it was cancerous. I sure do miss him.