What are the pros and cons of confronting a cheater directly versus ending the relationship without a confrontation?

What do I do if I (22M) just learned my best friend (23F) had a boyfriend for months, I've liked her for years, and I just don't feel comfortable looking for other women because there's very specific things and fears I can't trust people with?
How can consistent actions help you rebuild your reputation when dealing with a narcissist's narrative change?