still thinking - I'm late on this topic. I had a friend that did the same - sure we lived next door to each other so that made it very awekward. I just happend to be the better cook, she would just pop around and then it became the norm. Being on a tight budget did add to the stress. (I really don't mind cooking or sharing)
It was difficult but we had also been friends for years, I did bring it up and said that things were tight and though I didn't mind cooking and sharing if she wanted to continue to eat at my place we could split the costing or she could bring over some goodies too. It worked for us in the end - plus she didn't come around every night.
If you don't want to approach it that way - just say you aren't going to be cooking that night, and the next ... and the next ... and the next. Invite yourself around one night and jokingly say you've never tasted her cooking :) take dessert