Personally, I'd be inclined to book her in as soon as she shows signs of ceasing calling. If your friend isn't sure whether she got out during her call, it might be wise to ask the vet to carry on with the spey even if she's pregnant. I did this with Lucy, who turned out not to be pregnant. There was a small chance she could be.
Lottie, I'm sure you're aware that not mating an entire female often leads to ovarian cancer.