Like I said....you do need to have the conversation. Maybe this bill could be the catalyst but I just wouldn't go hand it to her and say I am not paying since you moved out.
Hey Mrs CB......you know this bill is due and I normally pay it out of this account. With that said I think we do need to discuss our finances moving forward now that we are separated.
FWIW...I paid all of our bills in our home. The only thing my W was concerned about is whether or not the debit card worked. I took the financial lead when she moved out and made sure that what I was paying and what she was paying was fair, equitable and something she agreed to. I make more than double what she does however I ensured she was responsible for her apt rent, living expenses, cell phone, groceries, gas. I do pay her car payment, car insurance, after school care for the kids and our family debt. I had to make a choice and that was to either pay it to set her free or refuse and hold her hostage in the home. I choose to set her free.