Hard to tell without more financial information, but generally speaking, having only one customer despite it being very happy with your product, is way too much

Hard to tell without more financial information, but generally speaking, having only one customer despite it being very happy with your product, is way too much risk to tolerate over the long run. This customer may be extending the startup's runway considerably, but assuming they leave or downgrade their spending with you, you'll be getting into trouble fast. Have an honest conversation with the founders about these potential facts. You'll learn with their behavior and response if you should stay or leave.