I have never encountered or heard of this behaviour. I would be rather startled if someone I had just met asked me to buy them a drink.

Lily Allen has. The relevant section:

Cut to the pub on our last night out,

Man at the bar cos it was his shout,

Clocks this bird and she looks OK,

Caught him looking and she walks his way,

"Alright darling, you gonna buy us a drink then?"

"Err no, but I was thinking of buying one for your friend.