That's a tough one. You approach her with all the info that was told to you and just refuse to say who told you.And if she keeps demanding who without admitting her faults then she's clearly cheating or cheated
first get her to try fess up, ask her if there is anything bothering her because you seem distant, if she says theres nothing , then say, well listen i found out you cheated on me and this lack of honesty just shows maybe you arent right for me. or if she says she cheated on you , try find out the reasons why and see if its worth working through!
casually bring up the the topic of fidelity...mention your sincere views and beliefs about those who commit infidelty....Let it go...this is what's called "planting the seed"...She will get to thinking...if she wants to talk about or not it's her choice...In the meantime find out the facts...maybe she did not cheat at all...people will spread lies and rumors for their own gain....you make the final decision start being grown...if you get involved with grown up matters you had best be ready to handle it ...otherwise it may be best not to have certain things in life until you are absolutely ready...peace