If your friend is Rhesus negative and her husband is Rhesus positive then after the first pregnancy she should have been given an anti-d injection which kills off any anti bodies created during the first pregnancy as the baby would be born Rhesus positive which is the dominant type. The anti-d does not always work which is what happenend to Mrs Nik and I, we had to have weekly checks in London from 20 weeks onwards and our youngest was delivered six weeks early, after some initial worries everything was fine and he is now a very happy 3 year old.
With regards to the flying I am fairly sure it shouldn't be a problem but they may be concerned if she is going to be away for a few weeks or more without the levels of antibodies being checked.