Which is what I would do. Politely ignore it.
And that means I'm uncomfortable. I don't tend to politely ignore things that I am comfortable with.
That said, I think we're missing the point. Why is the mum of a newborn in the office in the first place? She should be enjoying her many weeks (52 would be great) of paid maternity leave. And society should not penalize her for it, of course. That's the problem we should solve, not whether pumping/feeding at the office is polite or not.
