Friend is acting strangely?

My friends and I went to Dream World today and while we were at one of the train stations going there one of my friend’s pooed in her underwear, then acted as if nothing was wrong. I think she thought none of us knew except we all did but didn’t want to say anything. The strange part is I am sure she did it when we went to the toilet because while I was waiting for her to finish I could see what she was doing through the crack in the door. The other strange part is she kept wearing the same panties all day. Do you think I should talk to her or tell someone or should I just keep it to myself