Last year's Bluegrass song of the year was Del McCoury's version of "52 Vincent Black Lightning", a Richard Thompson Folk/Rock song. Your Bass player probably wouldn't have liked it because it wasn't done by Monroe, Flatt & Scruggs or the Stanleys. It sure sounded good to me and I would say that although it pushed the limits of Bluegrass, it was a Bluegrass song.

Blues is so closely tied to Bluegrass. As an example, I have heard Sittin' On Top of The World as a ripping Bluegrass song and slow Blues. I wonder if your friend would dismiss a slower version if the song played with Bluegrass instruments.

I tend to be a bit of a purist, The Stanleys, Flatt & Scruggs, Monroe et al., but it doesn't stop me from listening to Del McCoury, Rhonda Vincent etc.