Well, if her doctor is satisfied with it, then I guess that's ok. I know that the pill ' Microgynon 30' is known for this, so maybe it's that one.
I wouldn't like this, because surely you'd worry every month if you were pregnant? After all, the pill isn't 100% effective. Also, not wanting to be depressing or anything, but abnormal periods can be a sign of other "issues", so not having one could make spotting any future problems difficult.
If it were me, I'd probably change to a different pill.