There are, literary millions of causes of redness of the eyes. Without more information and/or a picture it is impossible to give you a meaningful answer.
From your post, it does not seem that your friend is having any symptoms that go along with his redness, so this is a good thing.
Hopefully it is from something benign such as allergies, a pinguecula
If you can obtain more information I will be happy to narrow things down for you.
I'm sure that you can understand how I am limited in what I can tell you about this issue when my only diagnostic "tool" is chat and there is so little hard data available to work with.
I realize that this was not the answer you were hoping to hear but it would be unfair to you and unprofessional of me were I provide you with anything less than truthful and honest information.
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