I understand that the main problem of your customer is that he wants CE. What you shall provide to your customer depends on the class of his product. There a 4 classes in CE: I, IIa, IIb, and III (see
meddev 2.4.1 guidelines edited by CE regulators for a comprehensive explanation). If his product is class I, the documentation shall be very light, compared to what it is for a class III product. Based on what you describe, your customer's product is an active device for diagnosis and might be class I as it delivers energy only in the form of visible light (warning: my suggestion doesn't replace a thorough evaluation).
But the CE classification is different from the 62304 classification. Yodon gave you some clues about 62304. Fortunately, there are similarities between CE classes and 62304 classes, as both are based on risk assessment. The software embeded in a medical device has the same constraints has the medical device. Similarly, based on what you describe, your controller might be class A (same warning as above), as if it has any bug or bad design, it might only change the highlighting of the patient's eye, without any injury (provided that the maximum intensity of the light is not dangerous).
If your software is class A, you are in good shape, you don't have so many things to do. You shall deliver a software development plan (how you are organized, what is your IDE), software specifications (what does it do), software verification (tests description and tests records), software release (which version is it, how was it built, what are the third party libraries), software maintenance (how do receive bugs, fix bugs and deliver patches or new versions) and risk management (this is the most complex if you never did it, your customer shall help you).
If your software is class B, you have more work to do. Most of times, software are class B, this category is a kind of "fall through" as class A is too light and class C is for seldom heavy cases where small manufacturers prefer not to rely on software to design their devices.
So, ask your customer what is the classification of his device, then buy ISO 62304 as yodon suggests.
I hope it helps.