My husband is trying to recall the name of a 50s sci-fi movie that had a creature that looked like a Christmas tree made of garbage bags.
It's a long shot, but any chance they're thinking of The Creeping Terror? It's from 1964, so not exactly the 50s, but the monster does look pretty awful and from some angles "Christmas tree made of garbage bags" fits.
(Image is from a review on Monsters A-Go-Go)
Whole movie is on YouTube, at around 40 minutes in you get a good look at the monster.
This is a bit of a stretch, but maybe "It Conquered the World"? The monster is sometimes described as looking like a giant cucumber, but its outline is conical.