My friend and a girl were talking and I heard her say to my friend "You stinker!"
What does this term mean?
Depending on the context, UrbanDictionary defines a stinker as:
A man who is charming, magnetic and often manipulative. Women's clothes fly off for a stinker and men will follow him into battle because he is just that cool. A stinker rarely gets caught with his hand in the cookie jar, but if he does, an intoxicating look or a clever remark gets him off scot free.
The term probably originates with a small child in diapers, where a "stinker" is obviously in need of changing. And, of course, if a child needs changing and several family members are available to perform the needed activities, there is a tendency to pass the child from one person to the next, avoiding being stuck with the task.
Thus a "stinker" is someone who, while probably loved, is considered unappealing at the moment, and apt to be avoided if possible.
But that was 75-100 years ago. The term has changed. The "smell" is now metaphorical -- the individual may say/do things that are less than "sweet smelling", but not obnoxious enough to result in a permanent disassociation from the person. They're a bit naughty, a bit of a tease, a bit obnoxious (but in a cute way).
From there the term can, of course, be adopted (depending on context) to have various sex-related meanings. The specific meaning of the term as you heard it is hard to guess, absent further context.
In British English, the exclamation "You stinker" was used by the literate, upper-middle classes to josh someone who had done something that was disagreeable or reprehensible. It was an idiomatic use of the word uttered among those of the same social class and was in common usage in the UK in the period between the wars and in the post-war period leading up to the 1960's. It sounds very P.G. Wodehouse in tone.