I want to buy gold for a friend, but would feel terrible if his account was banned.

More or less. Most important is you do normal 1m trades at a time. It's normal stuff because some furniture can cost millions or tens of millions. People tend to use it frequently.

Don't use market board or the mannequins.

IMO if your not doing over 50mil at a time it can be ok to trade gil, There are lots of venues that usually give daily/weekly giveaways that are worth around 5-20mil.
I wouldnt use marketboards or anything that will show an abnormality like trading a item that usually goes for 1mil for 50+mil.