Dear M.,
I agree with Sarah and Stephanie. I don't think smoking pot, in and of itself, is the end of the world, but I DO think this guy is NOT a responsible or appropriate parent.
There are two issues here, pot and parenting, and what's important is the way they interact.
First, pot is NOT good for you -- it kills brain cells, can damage memory if smoked in extreme abundance, and leads to extremely unhealthy eating habits. That said, it's not addictive, the claims that it's a "gateway drug" are scientifically dubious, and it probably does have medical benefits for people with severe illnesses (NOT healthy teenage boys).
BUT, what's significant here is that the dad is modeling antisocial, irresponsible behavior. Most teenagers don't have the capacity to distinguish between, say, a subtle critique of U.S. drug policy and a general injunction to go out and break all the rules because "society is bad." And if this guy is comfortable sending a "break all the rules" message to a teenage child, then, sorry, he shouldn't be in the business of being a parent.
I'd venture that this pot-smoking IS affecting your friend's son's grades -- not necessarily as a result of the pot itself (unless he's smoking it every day and then some) but because the son is probably applying his dad's "f*** the rules" attitude to school in general.
So yeah, I do agree with the moms who advise that your friend report her ex. Not because pot itself is the end of the world, but because this guy's parenting approach IS.
Best of luck,
Mira
P.S. This sounds like a very difficult time for your friend, but under the circumstances, it's wonderful that she has a great friend like you :)