Depending on her level of addiction and how many cigarettes she smokes a day, it could be more detrimental to the babies health to quit smoking.
While the go to answer for many is to just quit smoking during pregnancies, some doctors will advise that the mother cut down rather than stopping because the stress and symptoms of withdrawal are more harmful to both mother and baby. This isn't a popular response, especially in a society that is working against smoking period, but it does happen and is a case by case basis.
Smoking while pregnant increases risks, it doesn't guarantee them.
All in all, I would say this is a matter between her, her doctor and her family.
On a personal note, I find it appalling that someone, especially a woman, would make the statement of "they have plenty" in regards to how many children a person has and whether or not she should have more.
That's the mothers business, no one else's.