The company are within their rights, - but...
They are also obligated to have a "family friendly/flexible" policy, where work attendance requirements take into account individuals childcare needs.
I would suggest she pushes this with them, particularly now her hubby is self-employed, working long hours, trying to establish his business.
This does look to me very much that they have made alternative arrangements for employment whilst she was off and frankly, - I think an Employment Tribunal would find in her favour.
I'm an Operations Manager in a large organisation.