Discussing Modesty with Non-Muslims

Asalamu-alaikum! I have a question about how to explain to my friend that she does not dress modestly without sounding rude, or like I am calling her slutty:

The other day I was talking with a female friend, and I mentioned how I have some difficulty finding nice modest clothing in the local stores. She responded by saying "Well, I dress modestly and I haven't ever had a problem?" thing is, she was wearing a tank top and shorts that only covered about 1/3 of her thighs, and I was talking about my struggles with hijab. I wanted to explain to her that she is not modest, at least in Islamic terms, but I did not know how to without sounding rude. she genuinely seemed confused that I had issues, and she considered her dressing to be very modest.

What should I say if a future circumstance like this happens?