I guess the context might change things, but it really was long – here it goes. The gathering was meant to be low key, wear pjs, stay the night where we play a drinking game that holds a lot of nostalgia for us. I specifically told people that they should stay the night if they are going to take part in the games so that there are no worries about getting home safely. It was potluck style – I provided enough alcohol to cover the games and dessert. Others brought other forms of alcohol and snacks. This girl brought nothing.
An hour and a half before the party, I get at text from one of the girls who I am less close to but lives a 10 minute walk away informing me that she "may have pre gamed. Heavily." So I asked if someone was picking her up on their way to my house. She responds that she should be okay by the start time and that really she was only "momentarily tipsy." She tends to exaggerate for comedic effect so I took her at her word. Then she texts asking if she can come over early.

I told her that she could if she sobered up first because I didn't want to worry about her driving otherwise. Background: This person has a history of drunk driving - it is a pity she has never been caught because she seems to find it funny to relate stories of how she doesn't even remember how she got home last night. She is too cheap to take a taxi but refuses to adjust her drinking to fit her ability to drive herself home, i.e. she knows she's driving but she will drink as much people who are taking cabs because in her opinion "it isn't fair" that they get to drink more, even if she's committed to being the DD.
She later shows up at my apartment and first I'm relieved that she waited before coming over - then I realize as we chat and I'm finishing my food prep that she has actually spent her evening having multiple drinks at a bar before coming over. She is not only tipsy, but talking too loud and getting a bit emotional. I'm pretty angry at this point, because in my opinion, she should not have been driving. She then proceeds to eat a significant portion of the snacks I had put out for the party leaving me to worry that people were going to think that I didn't provide any snacks. Ex: A large bowl full of candy was eaten by the hand full until less than half was left by the time people arrived. Don't get me started on the Oreos. Her comments indicated that she was trying to soak up the alcohol.
Once people start arriving she drinks right alongside everyone even though she reveals she has at least a 4 drink head start and starts complaining that she is tired. Luckily she stops drinking (I was wondering whether I was going to have to cut her off even if I had told them they could stay the night rather than drive home) and switches to soda and seemed to be sobering up. When the last friend arrived late (she warned me ahead of time), she suggested we make a run to the grocery store before it closed and while we had a sober driver. Everyone agreed, the girl even suggested items we should get. Only problem is that she follows it up with saying that she doesn't actually want to come. She starts whining about how she just wants to stay in my apartment by herself while we pick up things. She actually lay down on my couch, grabbed a blanket and bundled herself up in it while insisting "I'm not going, I can stay here, just go." I try to insist because I am not comfortable with her being in my apartment alone. My other friends told her to just get up and come, I told her she could just stay in the car while the rest of us ran in if she wanted, but that I’m not leaving her in my apartment. It becomes clear that if I make an issue of it, it is going to be a Big Deal in the middle of what has otherwise been really fun for everyone so I drop it, but made it clear that I was not happy with the situation.
I couldn’t ask her to leave altogether because 1) in our group of friends it is unprecedented, 2) everyone would have seen it as an overreaction and 3) most importantly, I genuinely wouldn’t have felt comfortable letting her drive after she had been drinking. As we walked out, my friend Jenny (from other threads) pointed out that she could totally understand why I would be particularly uncomfortable leaving this person alone in my apartment in light of what had happened in the past with her. I had actually told this girl in other circumstances that I would never want to have her in my apartment alone. Another friend said that the girl was being ridiculous, that it was my apartment, and she should have gotten up and left when I asked her to. In the car, yet another friend revealed that this girl said something hurtful to her (while I was out of earshot or I would have intervened) earlier in the evening. We finished our shopping trip, returned to find her still on the couch, and continued the evening. As far as I know, she did not cause anyone any more problems.