My friend recently had his coins stolen on BFX. Given how it happened hackers can apparently drain your account with as little as control of your email.
sorry to hear that.
1. He had 2FA enabled
2. The ONLY thing the hackers needed was control of the email.
how did they log-in his account since it needs 2FA to log-in?
Presumably they started by gaining access to the email, searched that he had received emails from BFX before, reset his pw, then gained control of his account. Because he had 2FA enabled, they used a trading algo to make the worst trades possible matched up with their own personal account at BFX until all the coins were drained. For example they would trade BTC --> DRK and then trade the DRK --> BTC back at a slightly worse price matched with their own algo and doing thousands of trades until all the money is gone.
how long did this thousands of trades take that he didn't realize his account was compromised
Bitfinex has REFUSED TO REFUND his coins. So they are setting precedent that they will REFUSE TO REFUND YOUR COINS AS WELL. I would suggest staying away from them and say that this is one reason bitcoins will never ever become mainstream.
it is a very well known fact that you should never keep your coins at any exchanger. there has been a lot of hacks, alleged hacks, and scams that makes everybody think twice before considering keeping the coins at an exchanger.