It's an incredibly shit situation, because 6 months is a pretty awkward length of time to try and bring someone in for, we'll probably end up just trying to plod on being down 1.
These things happen, and I wouldn't be even slightly annoyed if it wasn't for the fact that she knew and took a short term role anyway, knowing she wouldn't be here for it. I think it should be considered fraudulent, frankly. If she just found out a few months in, shit happens, but this is a real kick in the teeth.
But it's illegal to ask if a woman is pregnant at the point of hiring, so I'm clearly a disgusting advocate for the patriarchy, and there's no problem with her taking a job she knew she wouldn't stick around for.