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.