It wasn't because it was sent email. It was because either your jpeg was bad or the program he used to open was incompatible. Try opening your saved jpeg in a program other than Illustrator. How does it look? What program is he using to open it?
as dlac said above, under file, go to save for web. it will options to save as jpeg, giff etc. double click on the file you just saved (save to desktop is easiest) and double click on it. your default program to view picture should open it up just as your friend will receive it.no program he was just opening as an email attachment