Breast milk production is based upon demand.
While some breast feeding purists will advocate strongly for using nothing but breast milk, I will encourage her to try to pump breast milk, increase her demand to appropriate levels and supplement if needed.
There was a similar post yesterday by a mother who wanted validation to stop nursing after 5 months because of the emotional and physical toll it is taking on her. The best thing the baby needs is its mother.
I was able to exclusively breast feed one child for a year while working 50-60 hours/week and pumping in my car multiple times per day. Exactly 2 years ago today, I was diagnosed with cancer just a few weeks postpartum (and the day before my son's 2nd birthday) and had to stop nursing at 12 weeks to begin chemo.
My daughter survived the formula experience with flying colors.
So, my opinion today is different than it would have been 2 years ago. I'd still encourage her to pump while she's at work and begin stockpiling breast milk (if possible). But, if it doesn't work for her or her daughter for whatever reason, her daughter will do very well with the formula supplements as well.
Wish her well in returning to work soon.