When to catch a cheater?

I'm 18 years old and about to go to college at the same school as my boyfriend. We planned to stay together. Recently, he met a girl, who, according to his best friend, he is talking to frequently and "seems to really like her". He's practicing guitar so they can learn together, go rock climbing together, etc. I know he's flirting with her, and I want to call him on it so we can break up. However, I'm not sure if I should wait until he actually cheats to break up with him. Also, I got all of my info from his best friend, and I don't want to drag his best friend into this. Do you think I should wait? And how should I tell him I know without incriminating his friend?

When to catch a cheater?
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