ok, Legally, if she has a sick note, the employer can start her maternity early, which isn't great because it means less time to spend with the baby.
BUT, if she can get signed off on the sick for an 'Unpregnancy related' illness, then this wont affect her maternity at all. Although I'm being signed off now (at 27 weeks due to pregnancy bloodpressure and sickness) but my boss is good enough to allow me to take it as sick leave for 4 weeks, and then I have 4 weeks holidays to take, THEN my maternity kicks in.
Hope this helps.