There's always an imbalance, or rather multiple imbalances, in any sort of relationship.
That doesn't mean to say that its necessarily abusive.
As for your friend, I obviously feel for her, but since we can never run the experiment where they didn't have the relationship it's hard to know how much the relationship messed with her versus how much she was already messed up versus how much she was messed up by her choice to be (hate the term) a sugar baby for a while versus how she was truly in love and was dumped.
At some point we have to take responsibility for the choices we make and legal adulthood would seem to be that watershed.
I can't handle the constant refrain of 16-21 are perfectly capable and fully rational when it's stuff they feel excluded from, but 18-25 are vulnerable and should be protected when it comes to bad choices they make.
Incidentally, the counter-argument might come from your one-time friend that (genuinely) he feels used because he thought it was love and feels conned after spending loads of money. Doubt he'd be getting much sympathy from most people though.
So, your anecdote is interesting, but not illuminating unless we hear both stories.