Just a little clarity: there is no "sober" state at this point. There is just the state when she hasn't started drinking yet or is hungover. Sober is when she has stopped drinking completely, the fog has lifted and she has begun the process of recovery: a week and then month and then years long process.
What she is when she is not actively drinking is not sober: she is just not drunk yet. Her mental attitude is still that of an active alcoholic.
Second, there are as many uninformed therapists who don't have a clue about alcoholism as there are fish in the sea.
Your therapist's suggestion to buy alcohol for an alcoholic is ludicrous. Fancy words like paradoxical intervention might work with people who have non-addicted forms of behavior problems, but addiction is in it's own category. The use of this technique is based on the mistaken idea that she falls into the range of typical problem behavior, ignoring her addiction. She is not suffering from a behavior problem, she is an alcoholic. Your therapist wouldn't be the first one trained in traditional therapy for average people...who doesn't have a clue what an alcoholic is.
That bottle will have no significance to her: she will think Yay: I don't have to go to the liquor store today! And, then,if you've bought high alcohol content booze for her, she will proceed to ruin whatever is left of the holidays.