Roommate “excused” From Lease By Landlord

I co-signed on a rental with my ex after we separated under the verbal agreement that her partner would be moving in and paying half of the lease when the partners prior lease expired. Due to my income I was made primary on the lease. The partner was added to my lease shortly after it renewed in 12/2024. They had a fight and the partner moved out this month. My landlord did not consult with me or gain my consent before sending her a letter advising that she remains on the lease but is not responsible for rent payments. Apparently, her new apartment complex needed the letter.

I have two main questions. 1. Can I sue the partner for her 1/3rd of the rent (assuming that would be all she legally owes with my ex and I both on the lease) since she is still on the lease and not contributing? Total rent is about $1900/month and there are 9 months remaining in the lease, so would this be too much for small claims court?

1. Is what the landlord did considered a breach of contract? They basically changed the terms of the lease without my consent. Can I use this as justification to break the lease or at least have my name entirely removed?

Also I am fully aware I was a fool to cosign. We have kids together and I wanted them to have a nice place to live when she has them. We did pick out a bigger place with the expectation that her partner would be living there with her kid as well. I tried to be supportive of my exs coming out/situation and wanted my kids to see how much I care, but this is too much. I’m hoping we can break the lease and get her a smaller place.

EDIT: So I called the landlord about the alleged letter and they claim they never sent one AND she is still active on the lease. It appears she may have lied to me so I would assume it’s all on me and my ex now. So at this point it seems like I should be able to sue her for her 1/3 of the 1900x 9 months left in the lease. So approx $5,500 ish not counting utilities which legally she owes even if she has moved out.

Do I have to wait until I suffer these damages or can I sue her now when she’s only missed one or two payments? And is this too much for small claims court? I’m in Indiana. Also Should I request that she continues to pay her share? So I am creating a paper trail?