What to say to a pal who is cheating on his girlfriend?

I have recently observed the starting signs of infidelity (e.g. holding hands). What should I say to my (male) friend? Should I intervene at all? Or allow fate to take its course and allow his girlfriend to eventually find out?

I am not one of those guys who high-fives his friends every time they "score." I sincerely care about my friend and wish to prevent the situation from becoming worse. Advice is much appreciated.

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