The general advice is that exercise is good in pregnancy, but that the mother should pay extra attention to how she is feeling. It's not recommended that mothers take up new activities in pregnancy.
More specifically on cycling, there is a slight risk of losing one's balance through hormone changes, muscles and tendons relaxing in preparation for the birth, and changes in weight and centre of gravity. Mrs. TI went through a stage were she would lose her balance unexpectedly, and only know that she was falling over because she could see the wall moving past her eyes. Her internal gyroscope would just stop working and she was unaware of it.
All pregnancies are individual and there are few hard-and-fast rules. Your friend will have to get used to listening to her body very attentively.
There are many pregnancy-specific yoga and exercise classes out there that she may be better off joining.