I've noticed that a friend of mine (age 40) has leukocoria in both eyes.?

Leukocoria is a white reflection from the retina of the eye, resembling eye shine. It is a medical sign of a number of sight and possibly life threatening eye conditions and generally does not occur in both eyes. Leukocoria can also resemble an advanced stage cataract, but then your friend would have severe vision problems. Are you quite sure that the shine you might be noticing in his eyes is not an intermittent reflection from lighting, eye glasses or contact lenses? Leukocoria is a very serious eye disease which is quite noticeable, expecially in the later stage, and your friend would have realized he had a problem long ago. His pupil would be white or opaque and his vision would be severely impaired.

If you are truly concerned, why not suggest that the two of you make an appointment with an ophthalmologist for a comprehensive dilated eye exam. (A dilated exam allows the eye doc to examine the back of the eye (retina etc.) Just express the idea as preventative eye maintenance. Most of us don't see an Ophthalmologist or Optometrist until we have a real problem, which might have been prevented or minimized if we had regular eye exams.