I learned that fella means something like friends. But this situation doesn't seem friendly, but is it okay to call someone by "fella"?
You are right, the word "fella" is normally used in friendly situations. While the situation here may not be friendly in itself, he is using the word "fella" to make it more friendly, because "fella" is a friendly and endearing term to use. The word "fella" can be used to make a situation more light-hearted. Ex: Fellas, I'm giving you a ticket for speeding but I won't ticket you for your expired insurance this time.
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