Of course, it is very normal. As your friends said, indeed, the color of eyes really can change. The reason is not your eyes itself, but because they are in different physical conditions. You can know, if you have studied physics, the change of colors is actually because of the refraction of light, color loosing and other physical phenomenon. So it can be said , you do not need to worry about completely.
Yes, it's normal for your eyes to change color. First, you should know that it's really not so much that the color is actually changing. Because such things are going with the color of your clothing, and your surroundings. And even the color of the sky will reflect and affect the perception of their color. This phenomenon is most noticeable in people with blue eyes. According to the eye doctor, people who have blue eyes are more reflective, because of the lower level of pigmentation in their eyes. So you can just pay more attention to see this, maybe you will discover it.
Although your friend has just turned from blue into green on eyes temporarily, he or she should better go to see the doctor and have a full check on eyes. There must be some imflammation for eyes or the body is lack of something nutrient. in a word, it is not normal.