@Cobus11
I am not sure what the outcome of this situation was, but a few lessons can be learnt for the future and perhaps helpful to other hosts:
1. Do not accept third party bookings, especially for long stays. You will not be protected. You will not be eligible for their insurance for any damages. They will not "have your back". I'm not saying that they have your back otherwise, but they have a very easy get out clause with third party bookings. This may be why they were less than helpful in this situation.
If someone is trying to make a third party booking, but it all seems pretty harmless, I just explain the policy to them and tell them that the person who wants to stay needs to book from their own account. End of story.
2. It sounds like in this situation, you knew it was a third party booking by the 'girlfriend'. In cases where you don't know until the guest shows up... Firstly, I have on my listing that guests may be required to provide ID on arrival, which I think can act as a deterrent to scam artists. If the guest who shows up is not the one that booked, I would definitely want to see that ID. Call Airbnb for a penalty free cancellation if you feel uncomfortable about the guest. Otherwise, I would also explain that third party bookings are against Airbnb policy and ask them to rebook with their own account, get the original booking cancelled and refund them for the first one.
3. Don't accept guests who only have a phone number or email verified. Require all the verifications. That's not to say you still won't get bad guests, but at least it reduces the risk.
4. I also ask plenty of questions to try to get a feel of what the guest might be like and, often there are red flags in their responses or lack of response! This is another reason to not accept third party bookings. You need to be corresponding with the person staying and they need to read and agree to your house rules. I have a question in my rules that guests must answer so that I know they have found the full version.
5. People have had different experiences with Airbnb CS but I have only had two instances where I decided the guests really needed to leave. Both times, Airbnb backed me up and spoke to the guests. The first time, they told the guests to follow my rules or leave without a refund. The second time, I just wanted the guest gone and they told her to leave, but I said I would refund any remaining nights, so that definitely made the process easier. Also, I was not asking for any money for damages.
6. Of course, if you ask a guest to leave or even bring them up on house rules, there is a risk of a negative, retaliatory review and it's close to impossible to have these removed. If you can handle the situation yourself amicably (some great suggestions from other hosts above) then great. I have found that bad guests often don't leave reviews, but it's always going to be a risk.
7. Leave an honest review for the guest (even if it turns out they weren't the ones who stayed as the booking guest is responsible anyway) but do not mention drugs or an Airbnb resolution case as these are grounds for removal of the review. If you are worried about a retaliatory review, wait to leave yours until close to the deadline. That way, the guest might be alerted too late to leave a review of their own. Of course, if they have already left a review, no need to wait.