Maybe you could offer her the names of some good babysitters for weekend? I've had clients ask me about weekends and I just say no. It's my familly time and I already work 50-60 hours / week, so it just makes it hard to balance everything out. It makes me a better provider when I get to have my own time as well
I just tell them that and offer some names and numbers of kids that I know are trustworthy to babysit for them.