Totally different catagories.
H1B attend to stay and attempt to
apply for green card.
J1 is a exchange visa and he has to go
back to the country of reidence
H1B can be renewed every 3 years up
to 6 years.
J1 has to be renewed every year.
H1B can be renewed in US.
J1 can not be renewed in US as far as
I know.
H1B pay tax
J1 does not pay tax
I am not sure if your friend can switch
from H1B to J1 either, because usually
the case is from J1 to H1B.
Go to Google, type H1B and J1.
I am sure you will find a site to sum up
the differences.