Is it possible that one drink a week could cause harm? Sure, it's possible. There are effects are called Fetal Alcohol Spectrum Disorders (FASDs) which are any negative effects caused by drinking, and are different from Fetal Alcohol Syndrome proper.
When some women get pregnant they do not know it, not at first. It can take several weeks until they learn about the pregnancy.
During that time they live their life. And that may involve smoking and drinking.
As long as your partner has stopped drinking and smoking since you've found out that your expecting, you're fine. At six weeks, your baby is still just a bundle of cells.
That's what my doctor told me when I first found out that I was pregnant.
Basically, don't feel bad because there's no harm done and she didn't know anyway. If it makes you feel any better, I used to smoke like a chimney I quit smoking at 6 weeks pregnant.