Is it an insult to call someone 'cabrón' in Mexico?
B: ¡Hola, cabrón!
No, it isn't exactly an insult in Mexico. It is just for a person with not a kind personality.
According to RAE definition:
Depends on context.
If it's your friend, there's nothing wrong because you are calling as a affectionate way, more or less.
For example, in Spain we have a famous corrupt treasurer called Luis Bárcenas, and in his party, the Popular Party, her colleagues calls him "Luis, el Cabrón".
But in fact it is an insult. Also a very hard one. If you don't have a much confidence with the person you should avoid it.
In México, cabron has different meanings. The first one from your example:
In this case it's just used as
There's also a third meaning, very common by the way, and it's used to refer to someone who is really good at something specific, for example:
These 3 meanings are used in México and I know them from my experience as a native Mexican.
Also can mean "difficult; hard to do" as in:
(Mexican - Guadalajara.)