I am TTCing right now and if someone asked me to wait 6 months it would be a HUGE deal. I cant say i want to get pregnant in a month and than BAM i'm pregnant. it doesnt work that way. It could take me a year or more of trying to get pregnant. You never know.
It seems like a lot of women travel pregnant with no problems. I'm sure if she's not too far along she can go. If i were you, I would just be happy for my friend and if my friend couldnt make my wedding, yeah i would be bummed for a few minutes but i would realize that my friends health and the health of her baby is really more important. I know when your a b2b you are focused on your wedding, how you want everything to be perfect, you want every detail planned out, and you know who you want there. But unfortunately it doesnt always work out that way, you have to adjust your plans a little. So count her as your moh and if she cant make it than you have someone else step in or you dont have a wedding party at all, you just have to roll with the punches. It may not be exactly what you pictured in your head but the main thing is that your marrying the man of your dreams.