Ethical question

This one is puzzling me, so I would appreciate your thoughts.

It is my normal practice that if I hear that a customer of one of any of my clients may be having trouble paying their invoices, I give my client(s) a heads up so they can take whatever action to mitigate the risk that they feel necessary. I feel this is part of my duty of care to my clients. However, in this case, the customer having difficulty is also one of my clients - in fact he owes me a tidy sum. So I'm torn between my duty of care (and duty of confidentiality) to the client in financial difficulty and my duty of care to my other clients who are suppliers to him.

So far, I've said nothing to anyone.

Self interest says, I should warn my clients - if he were to go under, he could take some of them with him and I could lose some really good clients. However, by warning them, they would probably reign in the credit they give to him and escalate what might be just a temporary cash flow issue.

Decisions, decisions