Why didn’t he say hi to me when he was with another girl? We’re just friends and he usually says hi?

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Let me clarify. He’s my neighbor and I’d say more of acquaintances but in no way it’s romantic.
Anyway I’m curious about etiquette in terms of greeting someone of the opposite sex while in a relationship.
Is it offensive? Why if that hurtful in any way to the SO? Especially if the other woman is classy and not the type to come on to a man?
Why didn’t he say hi to me when he was with another girl? We’re just friends and he usually says hi?
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