You're still missing the point I'm afraid. It was something that should have been cleared with the artist first and done outside of the environment of an auction, period.

Would you walk into a store, pick up some items then head to a cashier and say "You owe my boyfriend some money so pay for these for me"?

I hope the answer is "of course not."

While my metaphor is far from perfect, I hope it can get across to you the inappropriateness of the situation. An auction is something that is obviously done because the person wishes to raise money. That's the point of them. While I'm sure you meant no harm by it, I'm really not surprised that she picked someone else.