Colorado- Ex-common law partner filing tax return as Single when previously filed as married ?

My ex and I began filing as Married filing jointly in Colorado in 2011. We lived together as in signed a lease together and he was on my health insurance as spouse. We also have a child born in 2017. We never got married but I was told that filing taxes as “married” is basically a statement indicating you have to complete a legal divorce if you break up. I would not say we ever considered us to married. (We have separate accounts thank God)

We have been separated since 2023 and have not completed any legal divorce decree but (there is no way we are getting back together) we have been living apart since 2023 with both of us paying 100% for our own households. I also pay for childcare.

I just learned that my ex did his taxes late and decided to complete his 2023, 2024, and 2025 tax returns as single (he also itemized). I’m under the impression that you have to file Married filing separately until you get the legal decree? He is stating that his friend who “knows a lot about taxes” told him he can file as Single. Is this true? What do I do? Note I already filed my 2023 and 2024 as married filing separately.

Also, when I learned he itemized deductions, I went back and selected “itemized” as I thought you have to do this, and my amount owed jumped by a large amount without any further input on my end. Why is this? It doesn’t seem right because I can’t control him selecting the wrong information and I’m paying for it on my end.