I saw my friend wearing diapers? what would you do?

He is my friend. We know each other relatively for a long time. (about 1.5 years)
The other day when we went out to the local lake, I realized it at his waist when he was sitting on his bicycle (he leaned forward and his shirt rode up and I was coming behind him). I'm quite sure that I saw it right. Also when he crunched down n stuff. Note that he is not old, only 21.

I was just so surprised, but didn't dare to ask him about it. Would you mention it to him if such thing happened to you? I'm just wondering what could have happened to him.
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I saw my friend wearing diapers? what would you do?
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