It's (nearly) always proper to address someone as "buddy." It's a terrific casual way to refer to certain friends, acquaintances, or even strangers. As it's grown less gender-centric, I frequently use it to refer to female pals. Similarly, the term may be used to describe someone you dislike. I don't recommend doing so, but it's acceptable language. However, when you use this word to describe someone you're not related to in any way, it becomes more of an informal expression and can be considered somewhat disrespectful. I once heard someone call their friend a "mate" on TV and it sounded odd enough that I wondered if they meant it was a bad word. I never would have imagined so many people took it seriously enough to be offended by it. Only those who take things very seriously will get upset by this word. So, while it may be disrespectful to call someone a "mate," most people won't mind at all.