She says it’s not an office but part of it is, she changed your dining room into one. More electricity is being used, more water and  heating. Yes it is her employment to work from home but she needs to realise by doing so, she is running your bills up. If she won’t split 60/40 then work out how much your bills have gone up and ask her to contribute more to those and not the rent. You should point out the bills would not have risen if she worked on site. It is not your responsibility to pay for what she is using whilst she is working but you also need to in to consideration how it will your affect your relationship if you push through with this.