I believe it's the kind of scam where they build trust by letting him make esmall withdraws to make him invest a huge amount of money, and then they disappear.
Yeah. Its a pretty common tactic with shady exchanges these days - they reel you in with promises of big signup bonuses and other perks, as long as you deposit funds first. Then once you have put in a chunk of coin, surprise surprise, you find it impossible to withdraw anything. It preys on people's greed and gullibility.
I discussed all this with him yesterday, but he really wants to believe that it's not a scam. Let's see how it turns out.
Unfortunately, some people seem to be destined to learn the hard way. Your friend appears to be one of them.
Still, best advice is dont get fooled by appearances and any offer that looks too good to be true almost certainly is.