Erowid I have heard several people and spoken with lots of people also who have this opinion too - worldwide, not just from the States or in the UK.
Not advocating the use of it or otherwise, just thought another viewpoint worthwhile.
FWIW, I think have as many facts as you can, and not just from "Ask Frank" which is government sponsored. And in various places has more than a few holes in their knowledge.
I am more than aware this will probably cause a whole load of flack to come back at me for posting this. Again - I am neither advocating its use or condemning it. Just have all the facts so you can be fully aware of all the risks, and what you would consider an acceptable risk for you / your child.
If someone has been a pot-smoker for years then actually the stress the parent would go through during pregnancy in stopping smoking could cause more stress to the baby and them than if they continued to smoke it (even at a reduced rate).
My opinion - get as many facts as you can and then make up your own mind as to what its best for you / your child.
I've seen so many pregnancies where friendships have been trounced because of issues like this. EVERYONE is entitled to their viewpoint, but not everyone is entitled to make decisions on behalf of their friends. The choice the OP has here is not "How do I get my friend to stop smoking pot as I don't think its right", but actually - "How do I get my concerns over to my friend in a way that puts my point across, but lets her make her own choice in this?".
If you feel this strongly enough to stop being her friend because she doesn't agree with your point of view - there is your answer.
Help her have ALL the facts with this so she can make an educated decision rather than a possibly knee-jerk rebellious decision.
Other viewpoints and links are available
Cannabis-use-in-pregnancy
There are also many links out there that show it is Neuro-protective -that is, it can protect the brain rather than damage it.
Neuro-protective
Fox
NCBI
My background with this subject:-
Yes - I know A LOT about this subject, through personal relationships and also working relationships with clients.
No - I haven't smoked this myself during pregnancy.
Yes - I have had it in the past. Whilst I can see the benefits of it for medical use etc, its not really something I like all that much.
sits and waits for the flack...