It's karaoke, not Star Search

Where I go, they don’t let songs repeat. Just an idea.

Whatta whore! She sounds like she needs a good kick square in the nuts.

Well Hihorse, I really do think that you should follow at least some of the suggestions offered here. Show her what that whole “Do unto others…” thing is all about.
Unless of course, you still want to be her friend. In light of how she acts, if you still want to be friends, you can’t do anything. Very unsatisfying IMO. I say chalk it up to experience and let her have it. If she comes to you all broken down (unlikely, but you never know…) you can let her know that that’s what she was doing to everyone else. You might even change the wench, who knows?

I agree that this is a possibility. When a person moves to the dark side as quickly as this woman apparently has, and with as little thought, it might not take too much effort to bring them back.

[Alec Guinness voice]

Go forth Luke, and show her the power of the force.

[/Alec Guinness voice]

Hihorse: Did you offer any sympathy to the birthday girl? Not saying you had to, but it would have been a nice gesture.

Y’know, microphone cables are surprisingly easy to unplug. Especially if you’re in good with management.

Then you have only one option. Heckling! Show her no mercy. Ride her like a tired old horse then laugh and point and snicker. Not once, but every time. Involve as many other patrons as you can get.

She needs to know what it feels like. Only then will she see the damage she has done.

Write some Weird Al style parody lyrics about what a shitbag she is, set to the tune of a popular song at the bar. Request that song, go up and sing the parody lyrics REALLY FUCKING LOUDLY.

Ooh…to add on to tansu’s suggestion:

Use Shania Twain’s “That Don’t Impress Me” as the base song.

*You get on the stage
and belt out a tune.
You think that your voice
sends us to the moon.
You think you’re a diva,
drive us up the wall.
But you’re a regular ego-driven
bitch to all.

Yeah, you think you’re special.
Yeah, you think you’re such a star.

<spoken>So you’re a real bitch.

That don’t impress me much.
You have the 'tude, but do you have the guts?
Don’t get me wrong, I used to think you were nice.
But that was before you put your manners on ice.

That don’t impress me much.*