"Happy birthday." Formal v. familiar
I am male. I want to wish my female friend a happy birthday. Do I say "amica mia" or "amica mio?"
She is a former colleague and we were equals. We stay in touch regularly, mostly about work issues, and we have after-work drinks every 6 weeks or so. Nothing beyond that. Is "Buon compleanno amica mia/o, divertiti!" too familiar? Too formal?