Julie I completely understand being upset at your friend. The most upsetting part of it being that you may not have your best friend at your wedding. I think this is not a clear black and white issue and its a tough one. Here are some of my thoughts:
1. It's not cool that she keeps telling you "I'll be there no matter what", and then she seems not to be saving. At the very least she should be honest with you and explain the situation.
2. It sucks that GM lost his job and it seems like he really wants to be there and is doing his best to save up what he can. However, I agree with Tammy B that if you already offered the free trip to your friend I don't think you should now tell her that you will only be giving her half a trip. What I would do is have a heart to heart with her and tell her that while you want her there more than anything in the world, if she can't come you completely understand. And furthermore, you should explain to her that while you want her there, if she can't come there is someone that really, really wants to go and you would like to offer the free trip to them. So this way if she doesn't think she will be able to save the money, GM can have the free trip.
3. I have to agree with Erik on some things. Even though you have never put the guilt on her or forced her to come, people will always feel obligated no matter what, especially if she is your best friend. I agree with Erik that she probably just tells you what you want to hear but secretly she is complaining about the cost. I know your upset that she hasn't been saving and it stinks that she promised you that she would make it, but the reality of the matter is that she will make her own decisions as to how she spends her money. If she has "a little extra money" and wants to spend it on food or going out, or shopping or whatever, as much as it hurts, the decision is ultimately up to her.
4. As far as paying for everyone goes, if you had extra money I would say pay for both of them if you really want them there. But like you said times have been tough until recently and you don't want to go into debt b/c of this. Yes you want them there, but you also have to do what's best for you and FI.
I hope everything works out. Keep us posted.