I believe alcohol would be worse for the baby - particularly as it is consumed in a concentrated liquid form and crosses the placenta almost as soon as it is consumed.
Marijuana is (generally) sucked into the lungs, and whilst the substance still obviously enters the blood stream - I don't believe it would have the same intensity as the alcohol.
Having said that, both are definitely not safe to consume during pregnancy. I respect people's free choice too - but there is an innocent life at stake here. SHE may choose to consume alcohol and marijana - but the life inside her most certainly would never choose that - and she really does have a responsibility to take care of the baby first and foremost.
Perhaps you can print off/mail her/email her some articles on the short and long term effects that alcohol and marijuana consumption during pregnancy can have on the baby. Once these brain cells are lost, they cannot be regained and it will effect the baby for the rest of its life. Perhaps this will encourage her to view her behaviour in a different light. Perhaps you can also remind her that it is really a small sacrifice to avoid these substances for the duration of her pregnancy. Once the baby is born she can consume these substances in moderation without posing the same risk level to the baby. I think the younger you are, the longer 9 months seems. When she is older, she will look back and realise what an insignificant amount of time 9 months really was. I fear that although her lifestyle choices do not bother her now - when her child is grown and the effects of her former life are evident in her child (learning difficulties etc) - she may suffer with regret and remorse. No mother wants to feel this way. A bit of future perspective may go a long way. Having said that - you can't put an old head on young shoulders..... and some people choose to learn the hard way and we have to respect their choices - flawed as they may seem.