My friend have a small marketing agency and a client wants to pay only after seeing results. What would you do?

A friend of mine runs a small marketing agency, and he recently landed a potential client who likes the proposal. The only issue is that the client does not want to pay anything upfront they want to pay only after they start seeing results.

I can understand the client's perspective but I also feel it's risky for a small agency to spend weeks working without any guaranteed payment. I told my friend to be careful, but now we're both wondering what the right approach is.

For those of you who run agencies or service-based businesses. Do you ask for an advance, a partial payment, or ever agree to a results-based model?