I am a 23 year old with facial PWS (port wine stain) birthmark.
At first it sounded like your friend has a case of PWS, but the fading and darking does not normally happen. When angry, active, or anything else my PWS gets darker but it goes back to it's normal color.
Do you have any pictures of your friend that you can post here? If not and want to compare my pictures with his mark you can see them on my website www.hankspws.com
Since I am not a doctor I can not diagnose him properly. There is a couple things you can do.
First, go to the resource section at the top of this page and find a physician in your (or his) area or is driving distance and get an opinion from a specialist.
Second, go to the front of this website and ask one of the experts about this condition. Probably Dr. Liviten Ask the sergeon, or Dr. Nelson Ask the PWS expert would be your best bet to get. You will need to send pictures though because you do not know the extent of his condition.
If his condition is close to, or in either of his eyes he could have a very good chance of having glaucoma. If it turns out that he does have PWS he has even a higher risk of glaucoma. I strongly suggest talking him into getting his eyes checked out by a opthomologist in his area either way. Going blind (which I am but not from glaucoma) is even harder to deal with then haveing PWS. Glaucoma sneeks up on you with out even knowing you are loosing your vision. I know a few people that have lost their eyes to the condition so please convence him to get them checked.
If he ever wants to talk I am here for him, or if he wants to email me personally he can do that also. My email is in my profile. Birthmarks are hard to live with, but there are still ways to live a healthy normal life.
I hope this helps you and your friend.