I remembered a very good friend from the past. One day when we were chatting, he casually mentioned a girl, mocking her for almost being the same weight as her height.

I was silent for a few seconds and told my friend that I knew her. She has severe depression, and her weight gain was entirely a side effect of her antidepressant medication.
My friend immediately said, "Whoa, sorry, I didn't know."
After that, we gradually lost contact. Although he apologized at the time, everything changed from the moment he mocked her.