Spaying a dog in heat is usually more expensive. I say let her run this cycle and then get her spayed. Cheaper for them and its not going to hurt Zoey, since it sounds like she won't have access to a male.

BUT, on that note.....my friends dog was in heat and in their own backyard. They caught a male dog JUMPING their fence coming out of the yard!! Sure enough, she got pregnant. Male dogs can smell a dog in heat from a long ways away. So maybe mention this to them and just have them not leave Zoey in the backyard unattended.