Alcohol won't lower the effectiveness of birth control, it actually just gives you extra estrogen because there's estrogen in alcohol... That's why feelings are expressed more readily when drunk :lol:
your friend not taking her pill... There's a possibility of her being pregnant, but not an enormous one. It also depends when in the pack she is, but I wouldn't worry about it.
Advice... Your friend uses a back-up method for a week or so. No big deal really.