As a woman, you can call a female platonic friend a 'girlfriend' as most people will assume you are not a lesbian and you are talking about a friend.
If you are a teenager, you can call a male friend a 'boy friend' (note the space), but boyfriend (no space/pause) means usually a romantic relationship.
A male coworker can be one of your 'friends from work', or 'guy friend'. I guess a female coworker can be called a 'girlfriend' if you are good friends.
A lesbian will have a harder time explaining what she means by 'girlfriend'.