I've always called a lot of my female friends dude, or used "man" around them. Dudette when I was being silly (watch Indian shows & movies you see women calling each other like man come on yaar, or dude hurry up, Etc) , but this is the first I've heard of "guy".
As said above, see if he's using it with other women, it might just be part of his unique verbal makeup. However if it's just you... Somethings off.
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