Definition: Given a real-life anecdote of a complex ethical situation, generate a suitable title that describes the main event/root cause of the situation. Imagine that the given text is a social media post, and you have to write the title of the post, so the users can decide to read the main text or not.
Input: Throwaway as I know my flatmate browses.

**TL;DR** - moved into a new house with best friend, their SO is living with us and is meant to pay a third of the rent from 2nd month but at the last minute refused to. I said I wouldn't cover the shortfall and now (because of bullet 1 below) they are acting rudely toward me in my own home and I have been asked to just ignore it, but I don't feel I should - am I missing something?

I recently rented a new house with one of my best friends and we had agreed that their SO would move in with us after the first month (the SO had to give a month notice on their flat) and that we would pay an even share of all costs after that point (so it would be 1/3 from then on). I paid half of the fees, deposit and first month's rent and my friend paid the other half. SO moved in practically straight away and has been at the house 4+ nights a week. 

A few points for context:

* The house wasn't available right away and there was a month gap where I would have to find somewhere to stay, my best friend had a month left on their contract so I agreed to pay my them 2/3 of their rent costs and they stayed at their SO's place for the month (for free) so that we could offer on the new house together.

* The SO did not want to pay any deposit money or to be on the contract, which I'm fine with because couples have a tendency to break up and that would make things messy if they had paid money for the deposit.

It was a couple of days until we were due to pay the second month and my friend asks to speak to me and says that their SO has said it's not fair that they pay for the month as they technically wouldn't be "moving-in" until a few weeks after and then would be away over Christmas etc. As it turns out, the SO did not give notice in time on their current place and would be having to pay an extra month there. As an alternative the SO said that I should show gratitude to them (e.g. cover their rent) because I had stayed in my best friends room (which I paid them for!) whilst my best friend had stayed at the SO's place.

I said that if the SO was on the contract then this wouldn't even be up for debate and that, in any case, I shouldn't have to cover the cost of their mistake and that it wasn't fair for them to ask me to. We left it at that, and SO's costs have now been covered by best friend (which I still don't think is fair on them). 

I've now been warned by best friend that SO is royally pissed at me (too angry to even be reasoned with) because they think I am ungrateful and that I should expect but ignore rude behavior toward me. AITA for wanting to tell SO quite bluntly to give it a fucking rest and quit acting out? I don't think I should have to stay quiet, but I am worried there's something I'm missing here?

Edit- formatting

not covering rent when my housemate's so refused to pay their share