Client and former client – Feel like I am doing my client a disservice withholding what I know

There are 1 people:
S is my client
C is my former client

I was on a call with S (client). She is unhappy with her businesses performance and thinking of changing her business model

S said to me that she was in a membership which C runs. She said C is a high earner and runs her (spiritual) business in a way that S loves (from what she is experiencing)

S said that she wants a similar business model to C because clearly that is a model that works in this space as C is making lots of money from it.

I know that C is making very little money and in reality, not much more than S is making.

I felt I couldn’t tell S that for a few reasons:
* Shame that C is no longer a client and having to admit that to S (despite the reasoning being that C couldn’t afford my services since her business is not very financially healthy)
* Worried about breaking confidentiality and S thinking that if I speak about C and what her business is doing financially then I might also do that about S

However not telling S means I feel that I am doing her a disservice too.

I would love your views on what I should be doing with all this?!