Is it appropriate for a woman to bring a "Girl Friend" as a date to a wedding?

I would ask your co-worker, since you don't know the bride or groom, if it's acceptable. I personally wouldn't be offended if a female guest brought another woman, but that's just me. Also, I'm sure that you could tell your co-worker, or the bridal couple, that you will be going alone ... I'm sure they'll be happy to save on food for one more person. There's no law that says you have to bring anyone.