I know this is really delayed.
I'm 21years old and have CPKU, I am in a relationship and me and my guy have been together for almost a year now.
First of all I would like to say that the decision of being on or off diet is entirely up to your girlfriend and I am pleased that you choose to support her no matter what way things go.
Personally I have been on diet my whole life and have no intentions of going off because I know I want a family and I believe it will be easier for me to get to the correct blood levels if I haven't experinced what I'm missing out on, I also keep to my diet cos I know that my family has a history of addictions etc and I believe that food can be an addiction and I have already noticed that I tend to be an emotional eater, without the restriction of my PKU diet I think this could get much worse for me.
Anyways, my guy is completely supportive of me and my PKU, and he doesn't entirely get it either, he understands that sometimes I hate the diet (when I want to go out with him but have to walk out of 5 restaurants before finding somewhere I can eat), and sometimes I love it (when I'm experimenting with PKU baking and excited about the prospect of new food and new skills), and sometimes I am just not interested (when I have chocolate cravings XD), and it is also very hard to catch up to an entire life with the diet being a constant factor, i tend to forget that he hasn't experienced the diet forever.
Anyways to answer your question, your girlfriends Blonde hair and Blue eyes is a big indicator of PKU, however I don't think it indicates how much protien she can or can't have, it more indicates the affect the lack of phe that is being broken down into melanin which helps to provide hair colour and eye colour, most PKU children tend to be Blod haired and blue eyed at first, but as they stick to the diet and in particular take their formula thier hair will colour naturally as they get older, my hair is now a dark blonde, almost light brown from being almost white-blonde when i was little, and my older brothers is a medium-brown that sunbleaches.
I have to say that having kids tends to be a scary prospect for any mum, it is just more so for us with PKU as we don't want to harm our child by having high blood levels. Heck I am terrified of having kids and I've been on the diet the whole time! But please realise that if your girlfriend gets accidentally pregnant she can go on the diet as best she can and the child can still be healthy at the end.
Now I realise that you will want to get your girlfriend on diet in order to have kids, but I wan to point out that it won't happen unless she is entirely commited, and if you push her to do something she doesn't want she won't like you for it.
Lastly, as has been mentions there is now other treatment options, for one there is Kuvan which may or may not work for her, it all depends on the severity of her PKU and if she has some enzyme function, in some people it works and in some it doesn't. i'm not entirely sure why but from what I can gather it is dependant on if her enzymes that break down phe function somewhat or not, and even then it may not work. The company that provides Kuvan allows a free 30day trial to see if it works and then if it does they help to get insurance or some form of coverage for it (at least thats how it goes in the states I'm from NZ and we haven't got it yet), what Kuvan does if it works is it can generally double or even triple your protien intake.
That same company is also going through the clinical stages of bringing out and Injection drug called PEG-PAL, which brings down phe dramatically, I am not entirely sure on the details, but there is a Blog on here from a lady who has taken part in stage four of the clinical trail and it looks very exciting.
Anyways, good luck and I hope this has helped somewhat, if you have anymore questions feel free to ask