The most popular way of scamming is bitcoin double website. I don't know yet who invests in such websites. And those who invest in this type of website , they are very greedy. You say your friend has invested 3 bitcoins, I wonder why anyone who has the ability to invest 3 btc would invest in such a website? Maybe he could invest in greed or something else. However everyone should be careful of such websites as there is no chance of earning huge amount so easily. If someone offers something like this, it is for the purpose of scam.
As I know only some newcomers in online who knows only search on google or youtube the words "how to earn from online" are investing in these type of fishy Ponzi scheme sites. To be honest, I have also done it when I was a very newbie and don't know about bitcointalk, But I can not really realize that how a man who has 3 bitcoins can invest this big amount in this type of site? I really can not believe it. at least he should have a little knowledge about bitcoin investment. I surely believe that a person who has minimum knowledge about bitcoin investment and cares about his hard-worked money can not invest this big amount in Ponzi schemes.