I can feel that this is a tricky situation here and honestly there is a lot there to consider.
Firstly being 15, right or wrong it is very likely that this is a person your GF looked up to for quite a period of time, who has had a very deep and lasting impact on her life.. Do you remember falling in love at 15? Making sense of that? Feeling the heartbreak afterward & moving on? Age aside, this is a time where she likely had a lot of feelings, hormones and emotions. In this specific relationship with the large age difference she did likely feel a lot of comfort, protection, and safety “at times” and likely saw this person as dare I say a “father figure” in some ways and someone she could look up to admire, and trust.
Being 15 at this time we can likely establish this is not an every day situation, and yes actually illegal but whether it “should” have happened or not it did, and this will make up a huge part of her life and experience, and who she is today, as well as who she become in the future.
I am not going to go any deeper into the rights or wrongs or the potential situation, and depth of guessing how this came about, who the person was but we can establish that at 15 we do not have the brain capacity or foresight to protect ourselves through these types of situations, as parents we need to teach and protect our kids.
So what I will say on your question is;
Whether or not you “agree” with your girlfriends past you can not change it…. & her experiences help make her the person you love today. I can understand how seeing this, and thinking about it would make you feel uncomfortable, sick, and angry as you had mentioned to me personally however what your girlfriend needs is acceptance of where she has been and support for where she is going.
This is a relationship which will likely have ever going layers of depth and learning for her for many years to come, it would have made up a huge part of how she sees the world, and other people or men, and there was likely a lot of alternative factors that bought her to here in the first place.
I would consider the way you bring this up, it’s fair for you to feel uncomfortable, you want to protect her, you love her, the thought that she may have been taken advantage of, was not protected, or that she can’t see that would be frustrating but you need to approach this with love and care.
Don’t approach this with frustration, anger, pointing fingers, telling her what she felt was wrong.
You need to approach with love, care, understanding, not blame… Let her know you love her, your here and you care, accept what has already happened and know that it does make up a big part of her life, and it is going to come up from time to time. Butt let her know it makes you uncomfortable to see, and you find it hard knowing she still holds this so close to her….
Step back from your opinions and consider how she felt, and that like it or not this feels important to her, before you step forward and speak to her about it. You may find that approaching her in a different light to let her know how you feel about the future, not the past could make all the difference.
In terms of how to move past this, hopefully with these steps the 2 of your can find a way to communicate where neither of you need to attack or have walls up so you can move forward together, with that and open, caring, compassion communication toward BOTH of you and how you each feel in this situation, you should over time be able to understand each others perspective and try to respect where each other are at.
Give each other love and support to grow into the future, and care & compassion to learn from the past over time.
Also please note very importantly; if you girlfriend does feel ongoing affects of this later in life, or grows to realise more and more as time goes on encourage her to reach out, to talk, to seek help, as well as you, her family, and those around her - we are always more than happy to offer referrals to professionals more suited to helping heal specific traumas in life.