Lambert's fate at some point was a cause of many debates among fans, as it was never explicitly shown what happened to her. The beauty of it was that one could imagine just about anything horrible that their imagination would feed them. The downside of it was that some fans really clung to their own vision as oppose to treating it for what it was, which was an individual interpretation. That sometimes caused some long and passionate debates.
Ron Shusset: 'People have read all kinds of things into it that we didn’t intend, not even subconsciously. (2004 doc)
Some even suggested some sort of sexual act done to Lambert, perhaps originating from the fact that the film is full of strong sexual metaphors and imagery, from the story to H. R. Giger's designs. Or perhaps it originated from Sigourney Weaver's own interpretations of the original Alien creature, who on few occasions attributed sexual interest to the beast. Ridley Scott himself was puzzled by such statements
Ridley Scott: I never thought about it that way. I find that her comment is ... certainly odd (via alienseries blog)
The rape theory could also originate from the imagery of the tail going between Lambert's legs, but this shot was taken from Brett's death scene and from the full footage we know that the tail went between his legs and stabbed him in the back
Nowadays, with an easier access to the information, it has become clear what happened to Lambert in her last moments. First, both Veronica Cartwright and Scott confirmed she died of a heart attack, died of scare.
Scott: Veronica was always great at barely controlled terror. Catatonic terror. She's always like, two steps from a heart attack, which I think she finally does at the end - have a heart attack. 'You'd probably die before the thing touched you anyway. I mean, you'd have a heart attack, right? You'd turn and see it and last about four seconds before you had a coronary
The answer was given by both novelization and Dan O'Bannon himself
In the novelization also, its described that Ripley finds Lambert's body squeezed violently into the airshaft. This is what she is hanging from.
Dan O'Bannon set the record straight shortly after the film's release in November 1979 Famous Monster #159
O'Bannon: "Finally, returning to Lambert & Parker's murders, when we see the result of the creature's attack we notice that her bare legs are hung midair but we are not shown why. Well. the alien came thru the airshaft; that's where it shoved the body."
