Just heard back from my friend:

"I tried everything I know, and it always reverts to 524. This is interesting though. I clicked on the soloist track and put 406 in there and even IT would revert to 524 when it began to play. BUT I picked a tenor sax to see what it would do on something other than a guitar, and it held it just fine. I don't get it."

So, the problem seems to follow the song file.
Putting the 406 file into a "Soloist" track even gets changed to 524.
Picking something other than 406 (tenor sax, in her test) into Soloist track works...

If I click on "NEW", and select the =BRT_PBG style in the Style Picker, here is the description:

=BRT_PBG.STY. Light Rock w/ BossaPiano(140 RT)
Real and MIDI. Quintet.
RD: BOSSAROCK^6-a:Blastix, HiHat , b:Sidestick, HiHat Loud :Craig Scott
RT464: Bass, Acoustic, Bossa Ev 140, Neil Swainson
RT470: Piano, Acoustic, Rhythm Bossa Ev 140, Miles Black
RT406: Guitar, Acoustic, Strumming Ev 136, Jason Roller (**)
M50: Slow Strings


So, just to eliminate any other things. I closed BiaB.. Re-Opened it.

1) Create empty test-folder.
2) Select "New", and choose the =BRT_PBG style.
3) Give it a Title; Assign the start/end values (9-159); # of choruses = 1
....Check guitar-track setting = #406...OK!
4) Set key to Bb; set tempo to 174 (All of these settings are the values for the problem song)...
....Check guitar-track setting = #524... Ooops...

For awhile I tried creating it from scratch and then copy/pasting the chords in from the older version; then pulling up the "Bar Settings (F5) screen and clicking on the "Clear All Settings" button, thinking that would let me cheat... But even though it started out with 406/guitar-track, it would revert to 524...

With this last test, I didn't even copy/paste anything. I start with an empty track, for a couple of steps it's okay at #406... But at some point during the setup, it reverts to 524 before I can even start entering chords...

OK, I'm running out of ideas...

"You play pretty good--for a girl."