It is doctors on on-call rota's that I have experience of. For a full shift nursing rota I can't see why they can't accommodate, and certainly should be able to provide time for expression. Sometimes they won't allow people to work outside the usual shift pattern because missing hand overs can contribute to patient risk.
At 6 months though I think she has good grounds to push this. I think they use milk no longer being the main source of nutrition beyond 12 months as the reason it stops.
I was intending on returning BFing when Megan was 8 months, but it never happened.