Obviously totally unprofessional... but that doesn't mean it wasn't fun.

I've certainly seen FAs flirting with passengers, ignoring their work and other pax in the process... sometimes the passenger was clearly a previous acquaintance, sometimes apparently not. I remember one Alaska Airlines flight where a female FA was so hypnotized by a male passenger in the middle of the coach cabin, she sat on an armrest and draped herself across the seatset in front of him, chatting, until we were 30 seconds from touchdown.

I have read (probably apocryphal) stories of FAs hooking up with passengers, usually during long over-water night flights, but never seen it actually happen. Looks like you did.