AITA because I didn't warn my friend that she had pit stains ?

I (20f) went the gym with my friend (22f). When we were recovering from our workout, my friend started talking to his guy who looked early 20s. He had muscles but it didn't seem like he was there to workout on that day.

When my friend and I went to the bathroom, she looked shocked by her own appearance in the mirror. She said she looks terrible and asked me why I didn't her that she had pit stains. I just said she's at the gym. She said that I'm not a good friend since she wouldn't allow a friend to embarrass herself in front of a guy she likes. Am I the asshole ?