OP, friendships like this are poison for your soul and mental health.
Listen to your gut.
She is a user and you deserve better.
Decent people do not behave like this.
Send a message that unfortunately this has become a huge unexpected expense so it is better she invites someone else.
And then back away.
She may say ok no need to pay, but best to say no thanks, bring someone else, I'm no longer interested in going.
Screwing over someone doing you a favour is just the worst.
My friend had a friend that she was generous to, had given her weekends in her holiday home in a lovely spot etc. a couple of times a year, would have her for dinner as she's a great cook.
Her friend suggested they go to a concert together and she paid her for her for the ticket as you would expect to.
After the concert as they were leaving they ran into a colleague who stopped and raved about the concert and said how fabulous to get the tickets for free from their employer.
She turned a deep red and my friend was stunned and she turned and stared at her.
They left the venue and my friend said goodnight, they had planned to go for drinks and a bite.
She tried to say it was a misunderstanding but my friend never said a word, grabbed a taxi and went home.
She was so hurt, shocked and repulsed.
The full cost popped up on her phone refunded with an apology.
She never acknowledged it.
The friendship was over as far as she was concerned.
She did reach out and apologise profusely again, (but with no explanation really as to why she would do such a thing, she has a good job,)which my friend accepted but said that she just couldn't look at her the same way, so best they leave it.
Some people are just takers.
My friend is so lovely, but when she saw that behaviour in her friend she simply couldn't overlook it or want to be around it.
All she saw was someone who would screw a friend for £80.