Nope, she wasn't joking! I did tell her that I couldn't put my conception plans on hold for her wedding so we'll see what happens between now and August. I remembered when I told my dh about this. His response was 'WTF' and 'can we just not go to the wedding?' my dd is a flower girl at the wedding so we'll be going.
I think it was a case of my friend voicing her request before really thinking through what she was actually asking of me. She didn't seem too bothered by my response so hopefully she won't be too annoyed if I am pregnant on her wedding day.