As a fellow Aussie and knowing how bad our drink driving situation is here in OZ, I hope she didn't drive home.
I would not have allowed this at all but it's done now and you would never have dreamed she would behave this way, so I suggest not letting it happen again. I would be honest with this client and tell her how she behaved and impacted the service but of course, say it nicely.
I would gather you are a registered business and unless we hold a liquor license, we can't have liquor in a public business, so you could bring this up as a way to stop it from happening again and of course the insurance side of things too because you would not be lying, this is the truth, insurance would not allow this.
Imagine this......this client gets sloshed while you do her nails, you let her go home, she gets in car, smashes her car into another and kills 2 innocent kids 4 days before xmas and seriously injures herself, somebody who thinks suing is the best thing since sliced bread puts the seed in her head she could sue you because you knew she was over the limit but still allowed her to leave your place of business that shouldn't have allowed booze there in the first place and wham! you have yourself a law suit.
This is what could happen especially in todays society where people love to sue others and blame others for their own stupid mistakes.
It's just another angle to think about that's all.
There is a big difference between a client bringing a take away coffee from GJ's v's a bottle of wine from the bottelo
I know it's hard to say no, especially when new to the game but you must set the boundaries for your clients from the very first time they sit in that chair across from you.