Hi, i had gone through your question and understand your concern.
1)First thing i would like to say that in
pregnancy its better to avoid travel unless its very important.
2)Travel in second trimester of pregnancy by air is safe, if there is no related medical complication.
3) There is no increased risk of
miscarriage during air travel.
4) She needs fitness letter by her gynecologist.
5) She should do certain precautions- drink plenty of fluids, no too much coffee,tea, keep lower
limbs moving, frequent walks, belt should be properly fastened below abdomen, try to get aisle seat.
6) check guidelines and policy of your airline before travel.
Thanks i hope now you are relaxed and got your satisfactory answer.