You are in no way obligated to pay for her to come, but as you said you can afford to, you have to decide how much you want her there.
I am paying for my MOH and bridesmaids' accommodations. I made the decision knowing that many of them could afford to pay all of their own expenses, but I thought it would be a kind gesture. They will be responsible for their own travel costs. I also will be paying for accommodations and airfare for 3-4 close family members that could not financially afford to come otherwise but I really want to be there.
I have another friend that already told me she may not be able to come to my wedding because she is planning her own wedding. She is one of my best friends, but at this point I do not plan to offer to help her with expenses because I've made the decision to limit financial help to the bridal party and immediate family. Otherwise, helping others could get out of hand.
You said you really want your friend to be there and she really is struggling. I like the suggestion that someone else mentioned, have a candid talk with your friend and ask what she can afford. Perhaps if you pay part she could swing the rest.