The "I Can't Believe It's Not Butter" fiasco is in season 3, episode 6. You can find it on Netflix. Some other video services might carry it, as well.
The whole episode is loosely threaded with Ruby's addiction to butter spray, with clips of her therapist commenting on the number of bottles in the fridge, staged momentary panic at the grocery store when she can't find it, etc. At about the 23 min. mark, you hear one of her friend's saying, "Ruby's diet consists of one main ingredient, and that's butter spray." Then a very short montage of clips of egregious use start, while the voice over talks about being mindful of healthy cooking.
It may have come up in another episode, as well, but I don't remember. If you've never watched it, she's a great example of how not to lose weight. While you really do want her to be successful, her attempts at weight loss and bettering her health are generally cringeworthy, but probably very common mistakes.