You should take the costume back, but if you can't do that, then just make another one. Pinterest has thousands of easy, cheap, DIY costume ideas and I'm sure you can come up with something over the next two weeks.
Do you have other friends you can go out with on Halloween besides your BF's friends? Try getting a big group of people together, a mixture of his and your own friends, so you'll have some people there that you're comfortable with. Or invite your boyfriend over to your house after trick-or-treating and watch scary movies together or something. Keep in mind, Halloween is about having fun. That's the only goal of this holiday -- pure fun! So if you aren't having fun, it's pointless.
"Caring is not an advantage."