buy her drinks

To get no return on your investment in a girl

'Dude you hook up with that girl last night? You spent bank. She was all over you. Was she a bitchorslut?"
"She let me buy her drinks."
"Bitch."

buy her another drink

To pay a girl for keeping a man “company”
To pay a girl for sex
To pay for services

she is a good girl, that Patricia. Buy her another drink. She is a dirty little tart. He just needs some convincing.