I've gotta say .. YOU don't know how the "biz" works, do you?
I know from first-hand experience, because I work for a music publishing company. Our "biz" is, literally, publishing and selling sheet music and instructional books.
And I'll tell you from direct, first-hand knowledge, there is plenty of money to be made for the artist. Whether it's profit for them or still recouping advances from the record company, it doesn't matter. It's still profit for them.
Here's a quick example. Let's say we put out a book of Tom Petty transcriptions. Artist (or, more specifically, the songwriter(s)) typically make about 12% of artist folio books that we sell. If the book sells for, say, $20, that's $2.40 in Petty's pocket (for the most part --- he has co-writers occasionally) for every book sold.
How many books like that do we sell a year? Well let me look it up really quickly .....
Ok .. looks as though we're selling about 3,000 copies a year of Tom Petty's Greatest Hits tab book. So that's about $7,200 a year from that tab book. That's nothing, right?
However, that's one Tom Petty book. You know how many Petty books we have in our catalog that sell about that many numbers? A quick search I just did revealed about 15. So that's more like $108,000 ..... a year, for doing nothing at all except having written some of the most enduring hits in rock history.
And this is just tab books! We also have PVGs (piano/vocal/guitar arrangements), Guitar Chord Songbooks, and others in the Petty catalog that do well for him.
And if you're thinking ... yeah, well, he's an established artist, he's been around forever, I have 2 words for you: Hannah (frickin') Montana. Now granted, she's not making money off guitar tabs, but she's rolling it in by the boatloads in music books in general. It seems that we release one or two new HM books a week, each one selling off the shelves.
Or what abound a band like Maroon 5? They're a big new band on the scene, like many other big new bands. Their matching tab folio to the Songs About Jane album sold over 18,000 copies in two years. That's about $43,000 to the songwriter(s) already. I don't know about you, but I wouldn't sneeze at that.
As I said before, whether or not this money is going into their pockets or back to the record companies to recoup doesn't matter. It's still their money, and it's nothing to sneeze at.
Besides, just because your friend obviously didn't sign a good deal with the company and/or wasn't smart with their advances doesn't mean everyone's like that. You owe the record company what you owe by contractual obligation. If it states in your contract that you owe them for all the promotion, air travel, limo rides, expensive dinners, and other luxaries, then all of that expense comes out of your earnings. But there are bands that are smarter than that and aren't so wasteful with their money.
The record companies lose a lot on artists too that don't go anywhere. If you're $200,000 in the hole after 3 albums and the record company drops you, they eat that cost. The band doesn't pay it back.
Anyway, that's not my point. The point is that to say bands don't make money on music books is ignorant and uninformed.