Buying 2nd copy of the game
Yup, you can have two accounts (even on the same hard drive). As long as you’re not using a program to run both accounts YOURSELF at the same time to do something suspicious, you have nothing to worry about.
If you’re taking turns playing (which is what it sounds like, unless you do something wonky to run two clients simultaneously on one computer, which might possibly lead to getting flagged for a ban), then you don’t even need to install a second copy afaik. You do need to buy the second copy, but that’s so he can have an account. He just needs to log in with his account on a machine with the client installed.
Yes, just use the same program but let him log into his own account.