WIBTA if I don’t return the deposit?

I was living in a 2bed apartment & using the second bedroom as my office. Then I got married (this summer) & started spending more time in the suburbs where my husband lived. I still like to visit friends in downtown on weekends so we decided to keep the place until the end of the year but rent out the second bedroom. At the same time, my friend (who I’ve known since high school) was looking for a place so it felt like the perfect opportunity & I rented it out to her. We signed an agreement until the end of the year. Now, the landlord told me I have to move out by the end of the year. So this applies to her as well. In a way, it’s perfect timing because that’s how long our agreement was anyways. However, she found a place to rent starting Oct 1. This would leave 3 months where I will be responsible for her share of the rent.

In the city I live in, the rule is to give the last month rent as a deposit and provide 1 month notice for moving out. There’s no specific law for roommates though this is the general guideline people follow in this city. In usual scenario, I would not give back the last month rent since I did not get enough notice (just 10 days instead of 1 month). I already lowered the rent for her, and it’ll be more difficult for me to find someone else for just 3 months. I do understand the situation though and I’m fine with it if she does want to move out. But I don’t think I can send her the last month rent. That would be really just me taking a loss and making an exception because she is a friend.

The agreement we signed is until the end of the year and mentions after it’ll be monthly & she can give me 1 month notice to move out. Though I don’t think that applies right now since we’re still within the period of the agreement.

WIBTA if I kept that last month rent deposit to cover my loss?