How do I warn my friend about his relationship without being a douchebag?

Anonymous

I am worried about my friend and his relationship. I know his girlfriend personally, she is not a genuinely bad person and she is extremely intelligent and funny. However, she is promiscuous and untrustworthy. She had many exes, situationships, one-night-stands and she basically sleeps with any at least average-looking guy who hits on her. I know she microcheats on her boyfriends because she flirted with me when she was with her ex, she told me she is no longer attracted to him, she invited me to have drinks with her, just the two of us, told me I had 'sex appeal' and that I was ideal. Now, my desperate friend, who was rejected by all the girls he liked, lost his virginity to this girl, he is absolutely in love with her and wants to marry her. I am worried that he might suffer when she will start to flirt with other guys and stop being interested in him.

How do I warn my friend about his relationship without being a douchebag?
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