Be a Pal and Tell Her
Tell her! Not out loud in front of everybody else, obviously. But tell her in a discreet manner. She will be ever so grateful. You have no idea how embarrassing it is to walk around with a period stain without knowing, and then finding out that other people saw and didn’t tell you, just makes it much worse.
It’s like having a piece of lettuce stuck between your two front teeth and no one telling you. You would be really mad at people for not saying anything, right? This is the same thing (only worse). So don’t be shy, go ahead and tell her – just do it in a discreet, polite, and considerate way.