Should I tell my sister's boyfriend that she's cheating on him (with her ex)?

A little background: I'm 18 and my sister is 21. She has been in a relationship with her current boyfriend for about 6 months. She lives with him about 20 minutes from home.

What he doesn't know, is that whenever she comes home to visit, she spends most of the time with her ex boyfriend.

Last Friday, I caught her and her ex "fooling around". I feel bad for her boyfriend, and very guilty for not saying something. I've been thinking of messaging him to inform him. Should I? If so, how should I go about it?

I've never had to deal with this type of situation, so I welcome all insights, advice and questions.

How would you feel if your girlfriend's younger sister told you about her affair?
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+1 y
I appreciate all of your opinions.
I will continue to think on it and figure out the best way to approach the situation. Thank you all for the honest advice! It has definitely helped.
Should I tell my sister's boyfriend that she's cheating on him (with her ex)?
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