Verified answer

You should remind him that you chose to share your apartment with one person you selected, not three, and that you do not appreciate him giving an effective stranger access to your property. She uses the resources, increases the noise level, and if she is there every night, you could be violating your lease. He is the one who is demonstrating the poor manners, so don't feel badly about calling him on it. The sooner the better so they don't get too deep into the habit.

You need to have a talk with your flatmate. This is your space too and he needs to respect that. And part of that would be spending quality time with his gf over at her place. Otherwise, if she's over so much, you might mention that she should start kicking in for the rent, utilities, food and so forth...

Yeah she needs to be respectful and not make so much noise. I think it might be time for you to look for your own place though. It sounds like things will most likely not get any better there for you.

Is this flat yours? Does he have a contract or a lease? We need more info. in order to give you proper advise.

It is just completely rude of his girlfriend altogether, who in their right mind would want to moan so loud that everyone can hear???