It's classic business wisdom that you need to have diversified sources of income, and not be too reliant on any one source. But it's not always true. It's all very well in theory, but in practice it's not always so easy. If you've a big client wanting you to do more and more work for them, it's hard to turn it down - and unless it's unprofitable work, it's probably silly to turn it down too.
Of course, it'd be great if all businesses could eliminate reliance upon one or two big customers. But, if you think about it, the majority of people don't really have diversified sources of income at all. Most employees are entirely reliant upon one "customer" - their employer. You probably wouldn't suggest to them that they go part-time and try and find a few other part-time jobs to try and reduce the risk of over-reliance on one income stream. What you might sensibly suggest is that they should try and build up some savings, so that they're not reliant on their pay on a month-to-month basis any more, and if their employer does go pop, or their role becomes redundant, it's not a disaster. And you might suggest that they make sure that they acquire, maintain and develop a broad set of skills, and that they keep in touch with people in their network and industry. That way, if and when they're looking for a new role, they'll have a big head start and people may even seek them out.
Those are steps that a sensible business owner can take too. If you have a great, profitable relationship with one or two customers, brilliant. Use it as an opportunity to save up money for the future (yours and that of your business). Try not to get out of touch with the skills and people that you're not working with right now. Some of our most profitable clients have had people telling them they need to diversify for ages, but in the meantime have put away enough money that if and when their main income source dries up they'll be under no pressure to work again, either soon or ever.