I always try to have 7 revenue streams coming in at one time in the event one of them stops or is low for a period of time. So, no, I would not recommend going full time on Steem but I wouldn't recommend going full time in any one revenue stream. Think about what would happen if the price of Steem dropped or your friend couldn't make enough money from posts and comments in order to pay their bills? Do they have something else to fall back on to support their lifestyle?