Flatmate's boy 'friend' moved in for sometime but now has been living for 3 months

We have a 3bhk, 3 girls arrangement, and that's what everyone agreed upon. We all have visitors coming in for short (2-3 days) as well as long (2-3 weeks) duration, friends, partners, parents, relatives etc. In the month of June, one of the flatmates mentioned casually that her 'boy' friend (who earlier used to come every night and leave every morning), is moving in for 'sometime' and will stay with her, till the time he finds a place/flat for himself. No one said anything, as we all know it's difficult to find house in Mumbai. It's been 3 months now, and he is still living here, I don't see him leaving anytime soon. Now they cook together, wash clothes together, and basically living-in with other flatmates in a 3bhk.

It's not like he is causing any trouble; he is polite and kesps to himself for the most part. But this isn't what was agreed upon when they decided to live together. The apartment that was meant to be shared equally among three had become something else entirely—a space shared by four.

How do I ask her that when is her friend leaving? I am bad at confrontations

EDIT : just now, some bank personnel came to our house for verification. I got to know that flatmate's bf has applied for some loan, and they wanted to verify the address, and he has given our flat address. I told the that person that he is staying her temporarily and is not related to anyone of us

EDIT 2: we finally had the required conversation! At first, she listened, but then she snapped, asking, "What's your problem? He stays in my room." She went on to say that she and the guy had decided that he could stay with her. My other flatmate and I were stunned. We asked, "Did it not occur to you to inform us that you made a permanent decision?" Her response was, "He stays in my room and doesn't bother you guys at all."

I then brought up the issue of the loan, and that’s when she flipped. She said, "My lease ends in September. Either both of us, or just him, will move out by then, so the loan will get sorted." This hit me hard, as I started wondering, Is this really her plan? To take out a loan and then leave?

I wanted to discuss the loan further, but she began shouting and accusing me, saying how the guy helped clean the house (with the domestic help) while the rest of us were away (which no one had asked for, by the way). She also blamed me for using the AC too much, claiming that's why the bills are so high. Then she called me dominant and said I couldn't handle requests. So, I asked her directly, "What is your request?" She had nothing to say in response, and just continued shouting, repeating that the guy would leave by the end of September.

At first, my flatmate and I thought about giving her a one-week deadline, but the way she shouted at us made us reconsider. We felt she had crossed our boundaries again, but we let it go. After she left, she even came back to yell some more, complaining about how we inconvenience her, and questioning why we care when our friends and boyfriends visit too.

We didn’t want to argue further, so the next day we texted her on WhatsApp, saying that if he doesn't leave by the end of September, we would inform the landlord. Her response was dismissive: "Escalate it to whoever you want if he doesn't leave."

So now, we’re just waiting for the month to end.