As someone who was formally diagnosed with Asperger's Syndrome as a child:
Firstly, be aware that the condition "Asperger's Syndrome" has been depreciated by the American Psychiatric Association upon the release of the DSM 5, which rolled it into one part of the spectrum of conditions referred to as a whole as "Autistic Spectrum Disorder".
"Aspie" as a term could be used in a friendly way, but it can also be used in a perjorative fashion as a way of mocking someone by implying that they're beneath you; it's a shortened "childlike" version of the word, that could in turn be used to imply that the person you're using it to describe is childlike as well.
Since one of the prerequisites of being diagnosed with ASD is a reduced level of social ability, and that includes a reduced ability to determine the social intentions of others, I'd recommend just avoiding using it entirely, since it's entirely possible that even if you mean it in a friendly way, the person you're using it towards may not be sure that that's what you meant, and interpret it as though you meant it hurtfully.
And, ugh, I hate using this language because it comes from the worst parts of the Left Wing of politics, but you also have to consider the power dynamics of privilege involved in this as a neurotypical appropriating a form of address developed by a disadvantaged group of people to refer to themselves. Just because you can see a group of people using a phrase to describe themselves doesn't automatically make it appropriate for a person with a position of relative power over them to refer to them that way; it's certainly not as bad as a white person calling a black person the n-word when its acceptable for black people to refer to each other as such, but the same sort of social construct can still be said to apply.
Additionally, because of the trend of some people self-diagnosing as "Asperger's Syndrome", it's possible that some people might take offense at being called "Aspies" because you're implying that they weren't formally diagnosed and they're just making it up.