They have been dating for almost 3 years and I know it will ruin the relationship but I feel like I should tell him and show him the proof.
When did she do it?
Based on what information are you certain of ruin, rather than coming to terms with it? Is it a strong commitment upon which they'd established their affiliation? Are you aware of any hardships other than this? Is he particularly prone to wrath? What's her perspective & plan with regard to her actions? Is there any reasonable chance the information might find its way to him without your intervention? Have they shared physical assets of any kind?
You should not mess with people's lives on a kneejerk reaction. She's flawed, so is everyone in one way or another. It's not enough to guess at the consequences of inaction reliably. At any rate, reserve the evidence in case it's crucial, don't just lay it on. And be aware that you may well not be his friend after this.
This is actually very hard. Some say that in order for you to keep your friendship, stay away from your friend's relationship. But the consequences here is that if your friend will know that you already knew it, it will turn into betrayal!
First, you can approach your friend and disclose it with terms without compromising your friendship.
Second, you can approach his girlfriend and let her admit her personally on her boyfriend.
Either way, there will always be consequences. It might not be now, but sooner this secret will get revealed and the effect will be worse if you don't do it now. hence, its always your call. Your his friend, and a friend knows what's best for his friend. Listen to your heart. Do the right thing.