Thank you all for your responses!
Let's call the co-tenants good tenant and bad tenant.
Good tenant commenced a lease in Feb. 2015 and had a old friend / co-worker move in at the end of that first year (Feb. 2016). I insisted the friend be added to the lease with a new lease going March 2016 - February 2017. With the good tenant having a great track record, I used her discretion to have a friend move in (hindsight 20/20) but she would have to be added to the lease.
Enter April 2016 when bad tenant (friend) gets combative and refuses to pay her share of rent to good tenant (who continued to pay in full to me, I didn't even know of their rent sharing agreement). Their disputes have escalated, there is visual evidence (recording) of bad tenant spitting in good tenants room, walking around cursing, etc. This is just one example of the rude and crude behavior that has been reported. But it's just been rude or disgusting behavior between the tenants.
Good tenant attempts to "evict" bad tenant, which if of course is not permissible and bad tenant hires an attorney on the effect. Good tenant wants/needs to get out of the single family house due to the hostile environment and hired an attorney who sent a letter to cure the environment (I was not providing a safe, peaceful, and enjoyable premise) or the good tenant can leave the property in good standing and also with the return of her security deposit. The bad tenant has basically said she is not leaving and has no money to pay. Good tenant can stand living with brought in co-tenant so there is the stalemate. So an eviction is going to happen no matter what.
My favorite outcome would be simply non-payment of rent at the first of next month and I can start to the eviction process. But the good tenant is trying to get out in good standing and apparently is willing to go to court on that behalf (something I want to avoid with the expense). My thought was to consider releasing the good tenant in exchange for the security and pet deposit and then to commence the eviction on the bad tenant...