"...but he's familiar with TB, thinks it's great and has friends here..." Well if that doesn't make him suspect, nothing will!!
A lot of good info here -I'll add what worked for me when I bought a bass and started playing at 34 without ever taking a lesson:
1. LISTEN!! A lot! As you are already doing, listen closely to tons of songs in various genres and listen for the bass...starting to play bass completely changed how I listened to and examined music, in a GOOD way.
2. Learn the fretboard. Did not see this one mentioned above. I played guitar for many years and never felt the need to learn which fret was which note - just learned chords. For bass it is essential. And liberating. Along with knowing the major and minor scale patterns in all positions, and the constant relationship of note positions all over the fretboard. Knowing the fretboard (even just the first 5 to 7 frets to start) will help you immensely when you play with others, and reading basic chord charts.
3. Play with others as often as possible. I played guitar by myself for many years, disregarding timing in an effort to perfectly finger every chord. I learned more, and progressed more, in my playing in the first 30 minutes playing with other people, because I had to forego perfect fingering to keep playing along with them - the song kept going. It was the first time I ever changed chords in proper time, even though my fingering was a mess that first time. COMPLETELY changed my playing approach for the better.
4. Over time you will learn some of what I call "go-to" moves - patterns that (almost) always work to embellish a line.
All the best to you!