I think it's just a compliment, not cheating.
But if there are other signs like talking to her a lot or behaves secretly, then there's really the reason for checking him.
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Well to be very honest. I'm not sure if he's cheating but you can't be so sure. I had an ex who made me so suspicious of her activities in a way that made me think she was cheating. I wasn't so sure if she was cheating on me until I met a pro ate investigator who I was referred to by my brother. I texted her via her mail sphinx.coderaven at gmail dot com and well, results came out that she was cheating on me with a man she called his cousin and all my suspicions became reality, i was almost not able to contain the pain, but id rather not be lied to and cheated on... I deserve better! I later used the proofs to file a divorce case against her

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It's very insulting to your partner to flirt with another person. It's not truly cheating but it's almost as bad!