You've probably heard this already, but it will happen when it happens. Unhelpful I know, but Mother Nature is rarely one to bow to peer pressure.
If she is underweight or on alot of meds, it may delay things. I know it did in my case, but it did happen in the end.
If she has to justify the delay to her friends (which I know happens), she could say its due to her meds and leave it at that.