Confusion with rental contract

Okay I will try my best to explain the situation, I am an international student in Koblenz. I took this apartment in the end of October 2022 along with 2 of my friends. So in the October 2023, one of my friend ( let's call him A) informed our landlord that he is vacating the place ( he did give 3 months prior notice ). He brought in another person ( let's call him B), who stayed with us till end of July 2024. He informed us that he is vacating the place on 31st July with no prior notice.
Now I and my other friend had to find another person for the place. When B left the place, he informed us that he did not pay the rent for June and July stating he was not given a contract ( which the owner said he will and never provided with one throughout the period of his stay) . Now our landlord comes everyday asking us to tell that guy ( B ) to pay the rent for the 2 months and also for August since he left suddenly.

More details,

1. Landlord did not return the deposit back to previous guy ( A) .

2. Recently got to know that he did not return the deposit to the previous tenants as well ( 3 girls). One of girl called and told me that he is not answering her calls and messages. It's been 2 years since they shifted. She also told that her letters getting returned from the address stated in the contract.

3. All of us pay our rent Separately.

4. It is so frustrating he keeps coming every day speaking about the same issue to us, before when we had problems with the house, he said he'll repair it and never bothered with it.

5. Our landlord keeps bothering us ( me and my friend) to inform A and B to figure a solution for the problem. Even when I told him that it does not involve us, he should speak with them directly and not to us every single time about the issue. He has their contact and address. Which he never listened and again comes and asks us.

Questions,

1. Who is at fault ? What could happen ? What can we do about it ?

2. Person B said he won't pay because he was not provided with the contract and he did not pay any deposit. But he stayed here and used the resources for those two months. Does he have a point in his stance?

Any help is appreciated. We don't want to get exploited because we are foreign students. I can't make the head or tail out of the problem. It is stressful for me and my friend. Since we pay our rents on time and still getting bothered.