Region Transfer doubts.
Yes, to transfer Worlds/Regions at no cost one must have no characters on the account.
No, it is not possible to have characters on different Worlds/Regions on the same account; to do so requires separate accounts.
Yes and no; one can change Worlds/Regions once every 7 days, either by deleting all characters to transfer at no cost, or by purchasing Gems to transfer with all characters intact.
Your friend could create two separate accounts, one in each Region, to be able to play in both.
Good luck.
This is why I originally got a second account.
One side note: When a free account is upgraded to HoT it gets one free transfer. There is no notification im game, but when doing it the price is 0 gems.