I've been on both sides of this debate.
Don't hate me for being honest, but when my daughters father and I had been together for about 3 mths, I had an affair with his cousin. It was completely alcohol related, and I was only 18, still no excuses I know. My partner found out about 18 mths later, after some friends let it slip. It took some work but our relationship survived. I think if he had found out when it happened, we might have split up, but because the affair was so long ago, he gave me a second chance. BUT....
When I was pregnant with our daughter, he started drinking again (he was a recovering alcoholic) and he cheated on me several times with several different women. The first time I let it go because of my own past mistake, but the second time I said enough was enough. Too Late - he gave me tricho (a form of chlamydia). Worst thing - the girl he caught it from was suposedly my best friend.
I stayed with him, and justified it by saying my daughter needs a father. But when she was 3 mths old, I decided he was setting a shocking example for her, and that I would never want a man to treat her the way he was treating me. I left him for good.
That was 1 year ago. He is now sober, and we are working on rebuilding our relationship from the ground up. We love each other, but both have alcohol addiction issues that get in the way sometimes.
In answer to your question, I don't know whether you should tell this girl or not. Some things to think about: are there children involvd, how long have they been together, and do you think they will stay together longterm. Tread very carefully as it could blow up in your face. You could find yourself being called a homewrecker for telling, or being shut out for not telling.
Good luck, and my thoughts are with you.
Mum to Maya Grace 02-03, Sienna & Mercedes 10-06