Many reasons, infection, disease, injury etc. The best thing for your friend to do is take her dragon to a qualified reptile vet for a checkup and treatment.
It is very hard to know any cause of the problem of your friend's dragon's eyes, when nothing is mentioned about it's care. Things like, UVB light, basking temp, substrate, what he is fed, how often, is he eating, how much is he eating, is he active, how long ago did his eyes start to get swollen. Was there any injury to the eyes, was any chemical spray sprayed around the dragon or it's cage (sprays such as bug spray, hair spray, floral/scent sprays etc) Was he outside at all? The age of the dragon...FOr any of us here to even suggest a possible cause of the swollen eyes, we need to know the history of the dragon and it's husbantry details. Even then we cannot know for sure, as most to none of us are certified vets, only a qualified vet examining the dragon in person can properly diagnose and treat any injury/illness/infection.
-----
PHLdyPayne