It's a less formal way of saying "bitch." Unless this person is a friend of yours, then I would consider it offensive to be called that. (Sometimes good friends will use the word with each other, but if she's calling you that without your permission, it's disrespectful.)
The English language has a long history of people using offensive, derragatory, crude, and even obscene language in an endearing manner toward their good friends without meaning anything offensive by it. These insults are usually in the form of, "Hey you ugly old so-and-so, why weren't you at the meeting this morning?" It is something you would say to a person only if you were SURE that the other person would not misunderstand and think you were insulting him.
Biatch is Hip-Hop (African-American) slang for bitch which is not a word I would ever use, but which I hear used more often than I'd like.
So, to answer your question, biatch (pronounced bee-yatch) is always slang, unfortunately common, and should never be used by non-native speakers because of the risk attached to using it inappropriately.