After reading this article on the existence of Marla Singer, I am fairly convinced she represents his guilt and remorse. It seems entirely plausible Marla Singer materializes to remind Jack he is exploiting these therapy groups. In fact, even when Marla enters she says, "This is cancer, right?" nobody responds.
In addition to the blog, there are three examples of where I can think Marla Singer's presence is suggested but not required.
After Tyler's confrontation to Marla, he follows her to the laundromat where she removes the clothes from the dryer and proceeds next-door to sell them at a thrift shop. Even though the two are having a conversation in front of the thrift-shop attendant, she does not interject and instead only remains silent with a puzzled look on her face about the talk of bowel cancer.
Does this suggest that overly-civilized and feminized Jack invents Marla, a symbol of his guilt, and steers her toward outlet for his disdain of the order of society? Marla is self-destructive, manipulative, and a kleptomaniac. Whereas later on, Jack invents a hyper-masculine Tyler who is outwardly destructive, blatantly honest and disdains materialism and thus has no need to steal.
After Jack's boss finds some Fight Club paraphernalia on the Xerox machine, Jack says:
I'd be very careful who I talked to about this. It sounds like someone dangerous wrote it... someone who might snap at any moment, stalking from office to office with an Armalite AR-10 Carbine-gas semiautomatic, bitterly pumping round after round into colleagues and co-workers. Might be someone you've known for years... somebody very close to you. Or, maybe you shouldn't be bringing me every little piece of trash you pick up.
Then the phone rings, and it's Marla. Since we already know Jack invents Tyler as a way for him to cope with his repression from civilization, it seems like picking up the phone and talking to an imaginary person would use the same set of coping skills that led him to invent Tyler.
In the diner after not having seen Marla for an extended period of time, she orders food:
"I'll have the clam chowder, the fried chicken with the baked potato with everything, and a chocolate chiffon pie."
If Marla isn't real, the waiter only responds to Jack's subsequent request for Clean food, please., by saying,
In that case, sir, may I advise against the lady eating the clam chowder?
We know the clam chowder at any of the restaurants is arguably the most contaminated food item. So, it would be logical for the waiter to recommend not eating clam chowder to anybody requesting clean food.
There are many possibilities as to why the waiter says the "lady" but it seems like this is the most damning to the theory of Marla's non-existence. The waiter may be attempting to allude to the fact that Jack isn't the same person as he is during Fight Club. But it may not be as bad as it seems at first glance, because the waiter doesn't use the pronoun she or her.
It is also acceptable to refer to a high-ranking woman to their face as "the lady".
Since Jack even thought to himself --
Even if I could tell someone they had a good fight, I wouldn't be talking to the same man. Who you were in fight club is not who you were in the rest of the world.
Perhaps this is a play on Jack's lack of masculinity outside of Fight Club, the same way drill sergeants call their recruits ladies. After all, Jack does not want to be a celebrity, he wants to be one of the masses along with his followers.
Is there any more evidence or counter evidence to the claim?