While it's obvious from anyone who reads my Hubs (not many...) that I don't agree with drugs, I'll help you out on this one.
Any drug that is not prescribed and being taken as an addiction can be harmful to your friend's body, and even more so to a fetus.
If your friend takes Subutex while pregnant, her baby will be born addicted to buprenorphine, and will likely go through some degree of NAS (Neonatal Abstinence Syndrome).
Studies indicate that over half of babies born to Subutex using mothers will suffer this infant withdrawal period to some degree. Many infants with NAS need only a few days of additional cuddling, attention and quiet – some infants will need treatment, such as with an opiate tincture.
Additionally, buprenorphine is passed through the milk to the infant, and the manufacturer of Subutex does not recommend that it be used by nursing mothers.
However, SAMHSA has looked at the issue and has said that since buprenorphine is not well absorbed in the stomach, babies getting it through breast milk seem minimally affected by it, and that Subutex using women can breastfeed.
I hope this is somewhat helpful.. All in all, if her dosage isn't crazily high, the baby will be born with a minor addiction. If she's taking WAY more than she should be (prescribed or not), she could possibly have a miscarriage.