Pus on nipples
becomes infected, it may ooze a fluid which is actually serum. I doubt if it's pus
I don't know long she has had it, but she should see a doctor. Warm water compresses a few times a day and then cover it with someting like butt-paste at night should relieve some of the symptoms. Use a mild soap and wash linens in all-free and clear. I hope she gets better soon.