Well, problem solved itself.

On my way home from work I had told my gf that I was leaning towards no, and she somewhat understood, but I told her I'd give a final answer about an hour before the show.

The girl who got sick said she didn't need to recoup the whole face value cost of the ticket.

My gf invited one of her friends who is still a student who otherwise wouldn't have been able to afford the show. That friend was uber stoked to join in.

I called my gf to say I didn't think it was a good idea, she told me about the student friend. Non issue. Basically the only source of the drama was the roomie's ex.

I like my gf.