I think its still some inconsistency in your jar.
I would suggest the following:
run the following command in order to track the content of your jar:
jar tf YOUR_JAR_FILE_COMES_HERE
After you ensure that all the packages are places correctly and the Main class really exists there
Double check your MANIFEST.MF file.
Once the main class is defined correctly here your friend should be able to run java -jar YOUR_JAR_FILE_COMES_HERE_AGAIN
Another concert that I would check, On windows file names are case insensitive, while on linux/unix its not the case, you'll have to check the all the uppercases/lowercases are set up correctly.
Hope this helps