A friend of mine was MOH in a destination wedding she did not attend. She will be standing by the bride's side when they show the video at the reception. That could be an option if she is unable to attend.
Possibly she is upset because she was not given the option to attend or not. If she is asked to be MOH, then she would feel expected to be there with no choice. If you sit down & talk about it you can probably work things out.
I've been in a lot of weddings. I think its really important to make your wedding party feel like VIP guests, not hired help. Not that you have done this, but it definitly happens all the time. Its fun to help with a wedding, but only if it feels like choice not a duty.
My plan is to either not have bridesmaids or to wait to see who books flights & then ask them to be in the wedding. They won't need matching dresses, so I can wait until it gets closer.
Good luck! Just don't lose a valuable friendship!