Got this off Cosmo's website.
“Your Boyfriend Is Cheating on You”
No friend wants to be the bearer of this bad news. But consider this: “Imagine how you’d feel if your boyfriend was involved with other people and you discovered your close friend was withholding that information from you,” suggests Dr. Amador. “Not only would you lose the trust you had for your partner, but now it’s also been severed with the friend.” Telling what you know is a sign of respect, adds Amador. Begin by asking her questions about her relationship. “How are you feeling about him?” “Do you sense that you can trust him?” Get her to explore her feelings and instincts. If she doesn’t already have her own suspicions, she’s bound to ask where you’re going with this. Take a deep breath and explain that you feel compelled to tell her something, even though you know it will upset her. “Be sure to explain your rationale as to why you’re sharing this piece of what’s certain to be devastating news,” says Schroeder. Be prepared: She may very well lash out at you once she hears it. If she does, remind her of your pure motives. Adds Dr. Amador, “The friendship will only be damaged if it wasn’t a strong friendship to begin with.”
Personally, I'd tell her. I would be more upset with my friends if they knew and didn't tell me, whether it was cheating or emails.