My friend will come back to California in 2 weeks. Is it legal if I ask him to buy Powerball tickets for me when he come back here from Tennessee?

unless you are undr18 years of age then there are no legal issues to be concerned with

i would be very careful about doing that.  How close are you two?  What if you win and he doesn't?  Make sure you sign the back of your ticket!!!

Sure you can - this would be no different than someone buying tickets for others as Xmas gifts.