I think that you might want to show your friend an episode that doesn't have so much violence. (I know, hard to find.) But when someone is introduced to One Piece, their first thought is going to be "Wow, such a violent and kiddy-looking sort of show. It's like for little 12-year-old boys." Maybe you'd like to show him that it's not about how the characters look and it's not always about the fighting and killer moves and weapons. Since he probably knows it's already full of violence you could maybe show him something having to do with Robin in the Enies Lobby arc? Or maybe the fight between Luffy and Usopp but explain why they're fighting? You could just explain to him that there's a deeper side in One Piece. Parts that really make you think.
But to get a friend really hooked, I would just show them amazing AMVs I've seen. xD They show many parts instead of just one. But they have to be ones that really show what One Piece is about.
This is one of my top favorites that my friend made. I LOVE Robin's part. It gets better and better and it got my friend to watch One Piece. It kind of summarizes the whole series and you could even explain some of the parts. Spoil him a little and he should want to know more.
Good luck!
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@Sefal:
this may seem weird but i'm trying to make my mom watch one piece. she loves shows that are good and i really think one piece would be great for her. the only problem is how should i hook her in 36 year old mom? she watched a few episodes with my 3 year old brother and said that this isn't a kid's show it's pretty serious for a cartoon. mind you she only saw the 1st 10 episodes where she didn't really pay much attention cuz my lil brother was really watching.
Believe it or not, I'm trying to get my parents to watch it. At least one arc. xD It's my only wish. I even told them when I die (if I do before them) that they have to watch it and see why I love it so much.
The arc I really want to show my mom is the Arlong Park arc. I think it's the most beautiful arc to show a mom because it really focuses on the connection between a mother (Bellemere) and her children (Nojiko and Nami). I think you should try showing her that one since it's very touching. :')