It’s a personal view, but I could never see that the kind of problem you are talking about is due to a lack of saline in someones lungs…
It’s about proper, global, full-body hydration which does take time and careful monitoring to establish. Saline nebs are sometimes suggested on ITU, but I’m never convinced; it’s a bit like moisturiser in the desert, if you get what I mean…
Proper hydration, with careful fluid balance and attention to urine output is the key; 30 mls an hour as a base, less than that over 3-4 hours would be a cause to review and either bump up the IV or look again for other causes.
Edit; and good pain relief to allow proper deep breathing and coughing.
If she has an epidural, unlikley because of the emergency surgery, then it’s probably not working too well, or if she has a PCA, she’s probably not able to use it effectively; maybe needs a look over by the pain team or on call anaesthetist or outreach team if you have one.